Journal Logo

Laboratory Rounds

Do Hemp Products Produce a Positive Urine Drug Screen for Marijuana?

Henderson, Robert MD

Author Information

    In recent years, there has been an increase in food and cosmetic products sold in the United States that are produced using hemp seeds. These products have been used by emergency department patients and in courts of law to explain positive urine tests for marijuana, but are these claims justified? Can hemp products explain positive urine drug screens for marijuana?

    Hemp is a product of the marijuana plant Cannabis sativa. It has been used for thousands of years to manufacture clothing and ropes. It also has been utilized recently for the treatment of chemotherapy-associated nausea and for the reduction of intraocular pressure in glaucoma.

    Cannabis sativa contains several resins called cannabinoids including marijuana's active ingredient, THC (delta-9 tetrahydrocannabinol). Most of the THC in the Cannabis plant is in acid forms1. These acids are converted to THC when heated (see Figure 1). It then undergoes hydroxylation in the liver to form an active metabolite, 11-hydroxy-delta-9 tetrahydrocannabinol. This hydroxy form is converted to the inactive metabolite, 11-nor-9-carboxy-delta-9 tetrahydrocannabinol, which is the detectable metabolite.

    Figure 1

    When inhaled, THC is immediately present in the blood with conversion to 11-nor-9-carboxy THC, occurring in minutes.2,3 Ingestion of the plant has little clinical effect because the THC is mostly in an inactive acid form. However, ingestion of the heated plant (i.e., baked goods, brownies) does result in elevated THC and metabolites, but absorption is slow and erratic. In addition, much of the THC is broken down upon absorption via first pass liver metabolism, which blunts its clinical effect. Dermal absorption also occurs, but is less efficient than other sources of exposure.


    The typical urine drug screen uses an immunoassay (IA) to detect 11-nor-9-carboxy THC at cutoff levels of 20 ug/L, 50 ug/L or 100 ug/L. The Substance Abuse and Mental Health Services Administration and the U.S. Department of Defense developed use 50 ug/L as a cutoff to define a positive urine screen for federal and military employees, respectively5,6.

    There is no generally accepted cutoff for emergency department patients. Unfortunately, the immunoassay also detects other cannabinoids, such as those in hemp products, that can result in positive tests without the presence of significant levels of 11-nor-9-carboxy THC.7 Therefore all positive tests should be confirmed with gas chromatography/mass spectrometry (GC/MS), which is specific for THC and its metabolites, and, therefore, specific for marijuana use.

    This confirmatory test is mandatory in certain occupational settings [see Table 1] including federal and military jobs, but is not typically performed in the ED. SAMHSA and the Department of Defense define a positive urine confirmation GC/MS as a level greater than 15 ug/L.

    Table 1

    Hemp Products

    The recent influx of food and cosmetic products that contain hemp raises the question of whether the ingestion or absorption of these products can produce cannabinoids levels that can be detected on a urine drug screen in the emergency department.8 In addition, there are concerns that the use of hemp products can increase the urine metabolite concentration to cutoff levels used by businesses to detect drug abuse.

    The first question to be answered is whether the products claiming to contain hemp seeds actually contain THC. Hemp seeds used in food products typically contain less than one percent total THC.9 However, THC content may depend more on production methods than on the THC concentration of the hemp seeds. Significant heating is required to convert THC acid to THC, and THC is easily filtered out of solution.

    For example, analysis of hemp seed-containing beer showed no detectable THC or metabolites because the seeds are filtered prior to mixture with the beer.10 Analysis of hemp tea revealed only a five percent conversion of THC acid to THC when prepared in the typical fashion (boiling for one minute). When boiled for 45 minutes, 95 percent of the THC acid A was converted to THC. When this tea was filtered with a paper filter, only one percent of the THC remained1.

    Thus, THC concentration is highly variable in hemp products. The hemp teas and hemp trail bars probably have the highest concentrations whereas beer and quickly prepared coffee and tea have very little THC. As expected, the concentrations of metabolites also are quite variable.

    Hemp Product Studies

    Steinagle, et al tested 20 volunteers with urine GC/MS who drank hemp tea. Seven of the volunteers had detectable 11-nor-9-carboxy-delta-9-THC urine levels though the highest level was only 8 ug/L, well below the SAMHSA and Department of Defense cutoffs11.

    Fortner, et al tested volunteers who ingested one to two snack bars produced with hemp seeds. They found that 24 of 108 screening IA tests detected metabolite levels greater than 50 ug/L.1 None of the subsequent confirmatory GC/MS tests detected any 11-nor-9-carboxy-delta-9-THC. They also performed serial urine testing on volunteers who ingested hemp seed cookies. Five of the 13 urine specimens tested exceeded the 50 ug/L cutoff, but only two had detectable levels (0.7 and 3.1 ug/L) via GC/MS, far below the SAMHSA and Department of Defense cutoff.

    Struempler, et al performed serial urine immunoassays on a volunteer who ingested three teaspoons of unheated hemp seed oil per day. After only four days, the immunoassay detected a level of 55 ug/L. Callaway, et al also studied a volunteer who ingested two teaspoons of uncooked hemp seed oil per day for one month. At the end of the study period, both the screening IA and confirmatory GC/MS exceeded the SAMHSA and Department of Defense cutoffs and remained positive for three additional days. The level detected by GC/MS ranged from 36–87 ug/L, well above the cutoff of 15 ug/L. They then studied a single ingestion of 24 g. This volunteer had detectable cannabinoids on urine IA screen for only 17 hours, but the concentration peaked well below the occupational cutoff.

    Costantino, et al studied volunteers who ingested three teaspoons of unheated hemp oil.1 Urine IA screen detected cannabinoids above the 50 ug/L cutoff in one-half of the volunteers at four and 24 hours, but none in all subjects at 48 hours. The confirmatory GC/MS concentrations were above 15 ug/L in one-half of the volunteers at eight and 24 hours and ranged from 0 to 79 ug/L.

    Products produced with hemp seeds have variable concentration of cannabinoids and ingestion may lead to positive screening and confirmatory urine testing both in the ED and in the workplace. Products that undergo prolonged heating during preparation without filtration such as hemp teas and hemp oil are more likely to elevate urine levels.


    1. ElSohly MA, Jones AB. Drug testing in the workplace: Could a positive test for one of the mandated drugs be for reasons other than illicit use of the drug? J Anal Tox 1995;19:450.
    2. Cone EJ, Huestis MA. Relating blood concentrations of tetrahydrocannabinol and metabolites to pharmacologic effects and time of marijuana usage. Therapeutic Drug Monitoring 1993;15:527.
    3. Huestis MA, Henningfield JE, Cone EJ. Blood Cannabinoids. Absorption of THC and formation of 11-OH THC and THCCOOH during and after smoking marijuana. J Anal Tox 1992;16: 276.
    4. Hahessy SR, Pierce A, Tormey WP. Urine cannabinoids and usage of legal hemp products. Clin Tox 1999;37:515.
      5. Department of Health and Human Services. Mandatory guidelines for federal workplace drug testing programs. Fed Regist 1991;59: 29903.
      6. Wingert WE. Lowering cutoffs for initial and confirmation testing for cocaine and marijuana: Large-scale study of effects on the rates of drug-positive results. Clin Chem 1997;43:100.
      7. Liu RH, et al. Selection of an appropriate initial cutoff concentration for workplace drug urinalysis – cannabis example. J Anal Tox 1994;18:65.
      8. Tormey WP. Consequences of using legal hemp products. Clin Tox 1999;37:899.
      9. Callaway JC, et al. A positive THC urinalysis from hemp (cannabis) seed oil. J Anal Tox 1997;21:319.
      10. Gibson CR, Williams RD, Browder RO. Analysis of Hempen ale for cannabinoids [letter]. J Anal Tox 1998;22:179.
      11. Steinagle GC, Upfal M. Concentration of marijuana metabolites in the urine after ingestion of hemp seed tea. J Occ Envir Med 1999;41:510.
      12. Fortner N, et al. Marijuana-positive urine test results from consumption of hemp seeds in food products. J Anal Tox 1997;21:476.
        13. Costantino A, Schwartz RH, Kaplan P. Hemp oil ingestion causes positive urine tests for delta-9 tetrahydrocannabinol carboxylic acid. J Anal Tox 1997;21:482.
          © 2001 Lippincott Williams & Wilkins, Inc.