YOUNG ADULTHOOD HAS BEEN CHARACTERIZED as a transition from adolescence accompanied by extreme physical, psychological, and emotional feelings and demands. This life stage is accompanied by intimate partnership formation, high sexual partner turnover, high rates of sexually transmitted infections (STI), and unwanted pregnancy. STI are disproportionately experienced by adolescents and young adults in developed countries.1 The epidemiology of STI among young adults in Asia has been less well documented.
In Singapore, a nation characterized by a late age of sexual debut, teenagers accounted for only 3.8% of reported cases, but the highest STI rate was found in young adults 20 to 29 years of age, with gonorrhea and Chlamydia detected most commonly (with women slightly more likely than men to be detected).2 Among Japanese college students, the prevalence of Chlamydia in young adults aged 18 to 31 was 8.3%, with equal rates found in women and men.3 A survey of university students 18 to 25 years of age in the capital region of South Korea found the prevalence of Chlamydia to be 8.4% and 10.6% among sexually active men and women, respectively, with only 1 case of gonorrhea found among a male student.4 The prevalence of Chlamydia and gonorrhea among 175 youth seen at homeless shelters (a presumably higher risk group) in Korea was 12.6% and 15.4%, respectively.5
Studies on STI prevalence have been reported from several surveys of young adults in Thailand over the past decade. In military recruits in 1991, the baseline prevalence of STI among a random sample of 21-year-old men from northern Thailand showed 29.2% had gonorrhea, 7.0% with syphilis, 9.2% reported nongonococcal urethritis, and 19.2% gave a history consistent with a diagnosis of chancroid.6 Incident STI in this cohort of over 2000 men in 1991–1993 were also high: 8.8 per 100 person-years (py) for gonorrhea, 1.8 per 100 py for syphilis, 3.9 per 100 py for nongonococcal urethritis, and 8.1 per 100 py for chancroid. Most incident infections were acquired through commercial sex. Other identified risks were inconsistent condom use, drug use, and low educational attainment.6 In a study of 479 young men attending vocational colleges in south Thailand (age not reported), Chlamydia was found in 4% and gonorrhea was detected in 0.2%; the cited risks associated with STI were sex with girlfriends and inconsistent condom use.7 Finally, a study of 1725 students aged 15 to 21 years attending vocational school in Chiang Rai (northern Thailand) included an interview and specimen testing for human immunodeficiency virus (HIV), Chlamydia, gonorrhea, and illicit drugs. Of the sexually experienced students (48% of men, 42% of women), HIV prevalence was 0.5% (0.3% overall) and 0.4% (0.3% overall) had gonorrhea, and 5% (2.8% overall) had Chlamydia. Twenty-nine percent gave a history of methamphetamine (MA) use.8,9
Since the 1990s, amphetamine-type stimulants (ATS), principally MA and ecstasy, have been adopted worldwide by young adults,10–12 with a concomitant rise in HIV incidence13–16 and STIs.17–20 The use of noninjected drugs and HIV risks has been primarily reported in the developed world,21–23 with sporadic reports from the developing world.24–28 ATS use is associated with enhanced sexual arousal, prolonged duration of sexual intercourse, and intensified orgasm.29–32 Heterosexual male ATS users report a greater number of female partners, less frequent condom use, and exchange of money or drugs for sex.29 Female ATS users are reported to engage in more acts of vaginal sex with a greater number of partners, both transactional and casual.29 Each of these risks has been linked to HIV and STI risk.33–38
There has been an epidemic rise in the use of MA among adolescents and young adults in Southeast Asia since the mid-1990s, especially in Thailand.25 Given the association between MA and high risk sexual behavior, young adults who use ATS frequently may be at risk for experiencing a greater burden of STI.
In 2005 we began a randomized controlled trial of a behavioral intervention to reduce HIV risks associated with MA use among young adults in an urban area in northern Thailand. This study examines STI prevalence and associated sexual and drug use risks, using data from the baseline assessment.
Materials and Methods
This study was conducted within a randomized peer outreach network trial conducted in Chiang Mai, Thailand. The aim of the trial was to compare the efficacy of a peer educator network-oriented intervention to a best practice group life skills curriculum to decrease MA use and sexual risks among young adults in Thailand. The age group of 18 to 25 years was found to be most in need of preventive interventions based on ethnographic research findings conducted before the development and implementation of this trial. Between April 2005 and June 2006, 1189 young adults aged 18 to 25 were screened for study eligibility. Index participants were primarily recruited from venues where MA users congregated using street outreach workers. As this was a peer network trial, index participants were required to bring at least one member of their drug or sex network (i.e., a person with whom they used drugs or had sex within the prior 3 months) for study enrollment. Index participants were eligible for the trial if they were between the ages of 18 and 25 at screening, used MA and had sex at least 3 times in the past 3 months, and were able to enroll at least one of their sex or drug network members in the study within 45 days of screening. Network members were eligible for enrollment if they were also between the ages of 18 and 25 at screening, used MA at least 3 times or had sex with the index participant at least 3 times in the last 3 months. Participants were excluded if they refused to have blood drawn or provide urine and hair for testing, if they were enrolled in another prevention study, or if they refused to provide locator information. The trial was approved by the Institutional Review Board at Johns Hopkins Bloomberg School of Public Health, the Human Experimentation Committee at the Research Institute for Health Sciences, Chiang Mai University, and the Institutional Review Board of the Ministry of Public Health in Thailand.
This analytic study sample includes 658 participants screened for enrollment who had at least one STI test result available for analysis (representing 53% of screened participants). The study sample consists of 516 index participants and 142 network members (41 drug, 43 sex, and 58 sex and drug network members).
The interviewer-administered survey included questions on the participant’s sociodemographic background, substance use history, sexual history, and involvement in the drug economy. The network survey included questions on the number and types of members involved in each participant’s social, sexual, and drug networks and measures of the interconnectedness of these network members.
Specimens collected included urine, hair, and serum. The trial did not have sufficient funds to perform STI testing on all specimens collected, and therefore priority was given to indexes and their sexual networks. Index participants were tested for Chlamydia, gonorrhea, trichomonas, hepatitis B virus (HBV), hepatitis C virus (HCV), syphilis, and herpes simplex virus, type 2 (HSV-2). Sexual network members were tested for the same STIs with the exception of gonorrhea and hepatitis C. The laboratory protocol was altered based on financial considerations halfway through the baseline evaluation to include Chlamydia and gonorrhea testing of drug network members. All participants were offered voluntary HIV counseling and testing.
Amplicor CT/NG PCR (Roche Molecular System, Inc., Branchburg, NJ) was used to detect gonorrhea and Chlamydia in first-void urine specimens. HBV was detected by screening for anti-HBs, hepatitis B surface antigen, and anti-HBc using the IMx AUSAB test, IMx HBsAg (v 2.0) test, and IMx core test (IMx system, Abbott Diagnostics, Germany), respectively. IMx anti-HCV test (Abbott Diagnostics) was used to detect HCV. Macro-Vue RPR Card Test (Becton Dickinson, Sparks, MD) and Serodia-TPPA (Fuji-rebio, Inc. Japan) were used to screen and confirm syphilis. HerpeSelect 2 ELISA Ig G (Focus Diagnostics, Cypress, CA) was used to detect IgG antibodies to HSV-2. HIV antibodies were detected using IMx HIV-1/HIV-2 III Plus tests (Abbott Diagnostics), with positive samples confirmed with Western blot using Genelabs Diagnostics HIV Blot 2.2 (Genelabs Technologies, Inc., Singapore). For trichomoniasis, the conventional culture method (Kupferburg Broth without agar) in urine was used. All visits involving STI tests were followed 1 month later by a visit where the participant received posttest counseling, test results, and referral for medical treatment as necessary.
Correlates and STI Outcome Measures
The variables of interest included demographic variables, such as age, gender, and occupation (student, employed and unemployed); sexual behaviors including condom use (at last sex), age of sexual debut (≤16 vs. ≥17 years), and number of sexual partners in the past 12 months (0, 1, or ≥2); drug use variables, such as frequency of recent (past 3 months) MA use (0–3 days a month, 1–3 days a week, 4+ days a week), alcohol binges (0–15 days vs. 16+ days of drunkenness in last 30 days), and use of other drugs besides MA (yes or no); lifetime arrest history (yes or no); and network characteristics, measured by the number of members who were involved in both MA and sexual networks (0 vs. ≥1). Although sex work was a variable of interest in relation to STIs, only 2% of participants reported any transactional sex in the prior 6 months, and therefore it was not pursued further in the analysis. Variables were categorized based on their observed distributions, following natural cutting points.
The outcome of interest was the presence or absence of any laboratory confirmed STI (other than HIV). Other outcomes, including bacterial STI (presence of Chlamydia or gonorrhea) and HSV-2, were also explored.
Proportions are reported for categorical variables and differences were tested using χ2 tests. Medians and interquartile ranges (IQR) are reported for continuous variables, and differences were tested using Wilcoxon rank sum tests. As differences in the prevalence of STIs were observed between men and women, separate analyses were performed by gender to identify gender-specific correlates of prevalent STIs. Because STIs were not rare, prevalence ratios were estimated using Poisson regression with robust variance to determine risk factors for prevalent STIs. Log binomial regression was attempted, but the models failed to converge. Multivariate analyses were conducted and adjusted prevalence ratios and 95% confidence intervals are reported. Variables that were associated (P <0.10) with prevalent STIs in univariate analysis were included in each multivariate model. All statistical analyses were conducted using SAS for Windows version 9.1 (SAS Institute Inc., Cary, NC).39
The study population was young, with a median age of 19 (interquartile range, 18–20) (Table 1). The majority of participants were Buddhist (97.1%) and Thai (99.2%). Many (63.8%) participants were living with their parents at the time of the survey and over one third were still in school. The overall level of education in this population was low as many of those working or unemployed had not completed high school. Most (83.3%) participants had a current sexual partner, though few were married (1.7%).
Differences by gender were observed. Men were significantly more likely to be single, working, and living with their parents, and less likely to have completed high school compared with women.
Prevalence of STIs and Risk Behaviors
Overall, 38% of participants had at least one laboratory confirmed STI (Table 2). Chlamydia was most prevalent (22%), followed by HSV-2 (8%), HBV (8%), gonorrhea (6%), HCV (2%), and trichomoniasis (0.2%); no syphilis was detected. Of participants (n = 568) who volunteered for HIV testing, only 6 (1.1%) tested positive for HIV. STI were more prevalent in women than in men, although this difference was only marginally significant (P = 0.07). Women were significantly more likely to test positive for HSV-2 (P <0.05) and Chlamydia (P <0.01), whereas men were significantly more likely to be HBV infected (P <0.01).
Risk behaviors were prevalent in this population and differed significantly by gender (Table 3). The majority of the population used MA at least once a week, and more frequent users were more likely to be men (P <0.01). The main reported route of administration of MA was by inhalation (98%), smoking crushed tablets. Men were more likely to report more frequent drunkenness in the past 30 days than did women (22.5% vs. 9.3% reported being intoxicated 16+ days in the prior month; P <0.01). Use of other drugs was common, especially among men (P <0.01), and consisted mainly of marijuana use. The majority had initiated sexual activity before the age of 17, with more men reporting a younger age at sexual debut than did women (P <0.01). Men also reported more sexual partners than women did, with 73.7% of men reporting ≥2 or more heterosexual partners in the last year when compared with 33.6% in women (P <0.01). However, same-sex behavior was more prevalent among women than among men (29% vs. 5.2%; P <0.01). Condom use at last sex was generally low and was reported more often by men than women (P <0.01). Men were also more likely to report having been arrested in the past year (P <0.01). The main reported reasons for the most recent arrest included fighting (21%), using drugs (19%), and carrying drugs (19%).
Behavioral Correlates of Prevalent STIs
Behavioral correlates of prevalent STIs differed by gender (Table 3). For women, univariate results showed higher STI prevalence with more frequent MA use; this exposure, however, disappeared in the multivariate model, where number of sexual partners remained the important risk factor for STI. In the multivariate analysis for women, those who were older, had more heterosexual partners in the last 12 months, and had a younger age at sexual debut were significantly more likely to have a prevalent STI. In addition, women who had been arrested in the past year were marginally more likely to have a prevalent STI when compared with women who had not been arrested. There was no association between condom use at last sex with prevalent STI for women or for reports of frequent intoxication.
For men, there was no association between the frequency of substance abuse (MA and alcohol intoxication) and STI in either univariate or multivariate analysis. The prevalence of any STI was significantly associated with the number of reported sexual partners among men. Men who were 20 years of age or older were marginally more likely to have a prevalent STI and age of sexual debut was not associated with prevalent STI. However, not using condoms at last sex, having at least one network member in both sex and drug networks, and having been arrested in the past year were significantly associated with prevalent STI among men in multivariate analysis.
We also considered types of STI separately (data not shown), including the more common bacterial STI and HSV-2. When assessing the correlates of bacterial STI (Chlamydia and gonorrhea), there was a significant gender difference, where women were more frequently found to have these diagnoses (P <0.01). After stratification by sex, increasing numbers of partners and high frequency of MA use were associated with prevalent STI for women, while increasing numbers of partners, not using condoms at last sex and enrolling both drug and sexual network members in the trial were associated with prevalent bacterial STI for men. For HSV-2, women were also found to have this diagnosis more often than men (P <0.05). After stratification by sex, older age, ever arrested, and younger age at sexual debut were associated with HSV-2 among women, whereas only arrest history was associated with prevalent HSV-2 for men.
Young adult MA users in Thailand experience a high burden of undetected STIs: 43% of women and 35% of men had a laboratory confirmed STI, many of which were asymptomatic chlamydial infections. Two reports from the Thai-US Collaboration on vocational school students in a neighboring province provide a direct comparison to our findings.8,9 The STI prevalence in the Chiang Rai study was 5%, as compared with 38% in this report. Although substance abuse in our sample was not associated with prevalent STI, the overall STI prevalence in this sample of active MA users was nearly 8 times that of a similarly aged sample in the same geographical region, suggesting a higher risk profile. In a study of ATS using HIV-uninfected and HIV-infected women in Brooklyn, NY, 27.8% had positive cultures for Neisseria gonorrhoeae, Chlamydia trachomatis, or Trichomonas vaginalis at baseline,37 somewhat lower than the rates in our Thai sample. There are insufficient Asian data on substance using young adults and STI to compare our findings.
The main correlates of prevalent STI in this study were sexual behaviors. As has been reported in North America, Australia and Europe,10,40,41 and in Asian young adults,3,4 a greater number of partners was associated in this Thai study with a higher prevalence of STI for both men and women. Older age (≥20 years) was also associated with increased STI prevalence, reflecting an accumulation of lifetime numbers of partners and increased potential for exposure to STI. Condom use was associated with lower STI prevalence among men, as has been reported elsewhere,4,6 but not among women. It is possible that our measure of condom use (“Did you use a condom the last time you had sex?”) did not adequately capture the context of women’s STI risk.
Substance abuse, either frequency of MA use or drunkenness, was not independently associated with prevalent STI in this study. It was not possible to examine the association of any MA use with STI, as the majority of participants had used MA in the past 3 months, reflecting study eligibility criteria. One study in Thailand among treatment-seeking substance abusers found that participants admitted for MA use were significantly more likely to have chlamydia compared with participants admitted for use of opiates.24 Another study among Thai women admitted for drug treatment found that those who had used MA were more likely to be infected with HIV.26 Most other studies of MA use and STI risk have been conducted among specific high-risk populations in developed countries, in particular men who have sex with men, and have found an association between ATS use and HIV, as well as other STI.10,42,43 MA use may influence HIV and STI risk through its association with high risk sexual behaviors, including increased numbers of sexual partners and decreased condom use.6,10,21 Higher numbers of partners are also associated with incorrect condom use while high, trading sex for money or drugs, and may reflect psychological and behavioral disinhibition. In this study, frequency of MA use was associated with higher numbers of reported sexual partners (data not shown), which may be in the causal pathway between MA use and STI, and may account for the lack of an effect of frequency of MA use on STI prevalence in the multivariate analysis.
Sexual practices among unmarried Thai adolescents and young adults have changed substantially over the past 2 decades. A national survey in 1990 reported that 13% of 15- to 19-year-old women were sexually experienced, compared with 34% of similarly aged men.44 In 1999, among 15- to 21-year-old vocational school students in Chiang Rai, 43% of women and 48% of men reported that they were sexually active.45 In a context of changing sexual norms, MA has become increasingly popular among young adults, alcohol-related problems are manifest (e.g., a very high proportion of participants reported frequent drunkenness in this study), greater numbers of sex partners, and reports of high risk sexual practices, including concurrent partnerships and inconsistent condom use are becoming increasingly common and may place young adults at particularly high risk for STI acquisition.
There are a number of important limitations that need to be considered in drawing conclusions from this study. First, only one-half of the participants who were screened for eligibility were tested for STI, thereby allowing for the possibility of selection bias. We did, however, include all index participants and their sexual network members in the sample tested, although only a minority of drug network members received STI testing. Thus, the results for the drug network members may not be representative and may be subject to unmeasured biases. Second, in the analysis we report on “any prevalent STI,” which considers all infections equally. Thus, we included bacterial, viral, and protozoal infections, which may represent both current and past infections. However, virtually no participants reported having any symptoms related to an STI on interview. Our analysis of risks associated with bacterial STI and HSV-2 showed similar findings as the results for “any prevalent STI,” but the smaller sample sizes precluded sufficient statistical power to draw firm conclusions with confidence. Our age restriction for indexes and network members (18–25 years) also limits the generalizability of our findings to a relatively narrow age range of young adults, and therefore may not be valid for older persons. Finally, all data were collected cross-sectionally, limiting our ability to draw causal inferences for identified correlates of prevalent STI.
Combined with low levels of condom use, a large proportion of multiple partners, and overlapping drug and sexual networks, these factors suggest that if HIV enters these sexual networks, transmission is likely to occur rapidly. Although HIV in this cohort is currently rare, all of the behavioral risk factors are present to renew the further spread of HIV in the future if HIV enters these dense social networks. Preventive interventions focused on increasing awareness of risks associated with unprotected sex as well as high rates of partner turnover are needed for this young population.
1. Berman S, Hein K. Adolescents and STDs. In: Holmes KK, Mardh P, Sparling P, et al., eds. Sexually Transmitted Disease, 3rd ed. New York: McGray-Hill, 1999.
2. Chan RK, Tan HH. Sexually transmitted infections in Singapore teenagers. Ann Acad Med Singapore 2003; 32:25–28.
3. Imai H, Shinohara H, Nakao H, et al. Prevalence and risk factors of asymptomatic chlamydial infection among students in Japan. Int J STD AIDS 2004; 15:408–414.
4. Lee SJ, Cho YH, Ha US, et al. Sexual behavior survey and screening for chlamydia and gonorrhea in university students in South Korea. Int J Urol 2005; 12:187–193.
5. Lee SJ, Cho YH, Kim CS, et al. Screening for Chlamydia and gonorrhea by strand displacement amplification in homeless adolescents attending youth shelters in Korea. J Korean Med Sci 2004; 19:495–500.
6. Celentano DD, Nelson KE, Suprasert S, et al. Epidemiologic risk factors for incident sexually transmitted diseases in young Thai men. Sex Transm Dis 1996; 23:198–205.
7. Chandeying V, Skov S, Duramad P, et al. The prevalence of urethral infections amongst asymptomatic young men in Hat Yai, southern Thailand. Int J STD AIDS 2000; 11:402–405.
8. van Griensven F, Supawitkul S, Kilmarx PH, et al. Rapid assessment of sexual behavior, drug use, human immunodeficiency virus, and sexually transmitted diseases in northern Thai youth using audio-computer-assisted self-interviewing and noninvasive specimen collection. Pediatrics 2001; 108:E13.
9. Paz-Bailey G, Kilmarx PH, Supawitkul S, et al. Risk factors for sexually transmitted diseases in northern Thai adolescents: An audio-computer-assisted self-interview with noninvasive specimen collection. Sex Transm Dis 2003; 30:320–326.
10. Drumright LN, Patterson TL, Strathdee SA. Club drugs as causal risk factors for HIV acquisition among men who have sex with men: a review. Subst Use Misuse 2006; 41:1551–1601.
11. Costa e Silva JA. Evidence-based analysis of the worldwide abuse of licit and illicit drugs. Hum Psychopharmacol 2002; 17:131–140.
12. WHO. Amphetamine-Like Stimulants: A Report From the WHO Meeting on Amphetamines, MDMA and Other Psychostimulants. Geneva: Substance Abuse Department, WHO, 1997.
13. Edlin BR, Irwin KL, Faruque S, et al. Intersecting epidemics—crack cocaine use and HIV infection among inner-city young adults. Multicenter Crack Cocaine and HIV Infection Study Team. N Engl J Med 1994; 331:1422–1427.
14. Bluthenthal RN, Kral AH, Gee L, et al. Trends in HIV seroprevalence and risk among gay and bisexual men who inject drugs in San Francisco, 1988 to 2000. J Acquir Immune Defic Syndr 2001; 28:264–269.
15. del Romero J, Castilla J, Garcia S, et al. Time trend in incidence of HIV seroconversion among homosexual men repeatedly tested in Madrid, 1988–2000. AIDS 2001; 15:1319–1321.
16. Kellogg TA, McFarland W, Perlman JL, et al. HIV incidence among repeat HIV testers at a county hospital, San Francisco, California, USA. J Acquir Immune Defic Syndr 2001; 28:59–64.
17. Increases in fluoroquinolone-resistant Neisseria gonorrhoeae
among men who have sex with men—United States, 2003, and revised recommendations for gonorrhea treatment, 2004. MMWR Morb Mortal Wkly Rep 2004; 53:335–338.
18. D’Souza G, Lee JH, Paffel JM. Outbreak of syphilis among men who have sex with men in Houston, Texas. Sex Transm Dis 2003; 30:872–873.
19. Rietmeijer CA, Patnaik JL, Judson FN, et al. Increases in gonorrhea and sexual risk behaviors among men who have sex with men: A 12-year trend analysis at the Denver Metro Health Clinic. Sex Transm Dis 2003; 30:562–567.
20. Wong W, Chaw JK, Kent CK, et al. Risk factors for early syphilis among gay and bisexual men seen in an STD clinic: San Francisco, 2002–2003. Sex Transm Dis 2005; 32:458–463.
21. Zule WA, Costenbader EC, Meyer WJ Jr, et al. Methamphetamine use and risky sexual behaviors during heterosexual encounters. Sex Transm Dis 2007; 34:689–694.
22. Chiasson MA, Stoneburner RL, Hildebrandt DS, et al. Heterosexual transmission of HIV-1 associated with the use of smokable freebase cocaine (crack). AIDS 1991; 5:1121–1126.
23. DeHovitz JA, Kelly P, Feldman J, et al. Sexually transmitted diseases, sexual behavior, and cocaine use in inner-city women. Am J Epidemiol 1994; 140:1125–1134.
24. Beyrer C, Razak MH, Jittiwutikarn J, et al. Methamphetamine users in northern Thailand: Changing demographics and risks for HIV and STD among treatment-seeking substance abusers. Int J STD AIDS 2004; 15:697–704.
25. Razak MH, Jittiwutikarn J, Suriyanon V, et al. HIV prevalence and risks among injection and noninjection drug users in northern Thailand: Need for comprehensive HIV prevention programs. J Acquir Immune Defic Syndr 2003; 33:259–266.
26. Srirak N, Kawichai S, Vongchak T, et al. HIV infection among female drug users in Northern Thailand. Drug Alcohol Depend 2005; 78:141–145.
27. Gomez MP, Kimball AM, Orlander H, et al. Epidemic crack cocaine use linked with epidemics of genital ulcer disease and heterosexual HIV infection in the Bahamas: Evidence of impact of prevention and control measures. Sex Transm Dis 2002; 29:259–264.
28. Cleghorn FR, Jack N, Murphy JR, et al. HIV-1 prevalence and risk factors among sexually transmitted disease clinic attenders in Trinidad. AIDS 1995; 9:389–394.
29. Molitor F, Ruiz JD, Flynn N, et al. Methamphetamine use and sexual and injection risk behaviors among out-of-treatment injection drug users. Am J Drug Alcohol Abuse 1999; 25:475–493.
30. Kall KI. Effects of amphetamine on sexual behavior of male i.v. drug users in Stockholm—A pilot study. AIDS Educ Prev 1992; 4:6–17.
31. Urbina A, Jones K. Crystal methamphetamine, its analogues, and HIV infection: Medical and psychiatric aspects of a new epidemic. Clin Infect Dis 2004; 38:890–894.
32. Klee H. HIV risks for women drug injectors: heroin and amphetamine users compared. Addiction 1993; 88:1055–1062.
33. McCoy CB, Lai S, Metsch LR, et al. Injection drug use and crack cocaine smoking: Independent and dual risk behaviors for HIV infection. Ann Epidemiol 2004; 14:535–542.
34. Kral AH, Bluthenthal RN, Booth RE, et al. HIV seroprevalence among street-recruited injection drug and crack cocaine users in 16 US municipalities. Am J Public Health 1998; 88:108–113.
35. Nuttbrock L, Rosenblum A, Magura S, et al. The association between cocaine use and HIV/STDs among soup kitchen attendees in New York City. J Acquir Immune Defic Syndr 2000; 25:86–91.
36. Hser YI, Chou CP, Hoffman V, et al. Cocaine use and high-risk sexual behavior among STD clinic patients. Sex Transm Dis 1999; 26:82–86.
37. Wilson TE, Minkoff H, DeHovitz J, et al. The relationship of cocaine use and human immunodeficiency virus serostatus to incident sexually transmitted diseases among women. Sex Transm Dis 1998; 25:70–75.
38. Ross MW, Hwang LY, Zack C, et al. Sexual risk behaviours and STIs in drug abuse treatment populations whose drug of choice is crack cocaine. Int J STD AIDS 2002; 13:769–774.
39. SAS [program]. 9.1 version. SAS Campus Drive, Cary, North Carolina: SAS Institute Inc., 2007.
40. Lorvick J, Martinez A, Gee L, et al. Sexual and injection risk among women who inject methamphetamine in San Francisco. J Urban Health 2006; 83:497–505.
41. Methamphetamine use and HIV risk behaviors among heterosexual men—preliminary results from five northern California counties, December 2001–November 2003. MMWR Morb Mortal Wkly Rep 2006; 55:273–277.
42. Buchacz K, McFarland W, Kellogg TA, et al. Amphetamine use is associated with increased HIV incidence among men who have sex with men in San Francisco. AIDS 2005; 19:1423–1424.
43. Hirshfield S, Remien RH, Walavalkar I, et al. Crystal methamphetamine use predicts incident STD infection among men who have sex with men recruited online: a nested case-control study. J Med Internet Res 2004; 6:e41.
44. Sittitrai W, Phanuphak P, Barry J, et al. Thai Sexual Behavior and Risk of HIV Infection: A Report of the 1990 Survey of Partner Relations and Risk of HIV Infection in Thailand. Bangkok: Thai Red Cross Society, 1992.
45. Liu A, Kilmarx P, Jenkins RA, et al. Sexual initiation, substance use, and sexual behavior and knowledge among vocational students in northern Thailand. Int Fam Plan Perspect 2006; 32:126–135.