The term urbanization has no universal definition. Broadly, urbanization refers to movement of people from rural areas to cities but can also refer to the expansion of urban boundaries and the development of new urban centres . Definitions of what constitutes an ‘urban’ area also vary. Countries may use criteria such as population density (e.g. a population of more than 50 000), facilities (e.g. higher level schools) and infrastructure (e.g. street lighting) in their classification . Urbanization in industrialized western countries has been ongoing and relatively well managed for the past 200 years. In resource poor regions such as Africa, South East Asia and South America accelerated mass urbanization over the last 50 years has often overwhelmed the city infrastructure and resulted in people living in adverse conditions [2,3].
Urbanization and its relationship to poor health including mental health have been well documented in the literature [2,4–6]. Health problems associated with urbanization are contingent upon various factors and there are complex pathways in the relationship between urbanization and mental health. Some of these are the pull and push forces driving migration, the economic status of the region and the adequacy of the urban infrastructure to cater for internal and external migrants . Adverse climates, natural disasters and civil war are common push factors for urbanization. Economic influences such as job opportunities, desire for education and better housing pull rural dwellers towards urban areas . However, the reality faced by migrants is often that of unemployment, homelessness, social isolation and cultural disintegration all of which could act along a pathway to a range of mental health conditions [4,5].
The Global Burden of Disease Study suggested that in 2013 alcohol and illicit drug use combined were the seventh largest risk factors for burden of disease. Alcohol and illicit drug use were associated with 3.163 million deaths globally, an increase of 51% since 2000. The increase was most notable for illicit drugs, which had a 225% increase in attributable deaths from 2000 to 2013 . Urbanization and the factors associated with it play a role in the changing patterns of substance use and addictive behaviours [8,9]. This does not preclude rural areas from also experiencing increases in substance abuse. Rural and urban spaces do not necessarily reflect distinct dichotomies but often exist on a continuum from one space to another . Additionally, rural areas also experience numerous drivers of ill health that is not within the aim or scope of this study. This paper will review recent studies that examined the relationship between substance use and urbanization in high, low and middle income countries. We also aim to highlight the areas for intervention.
URBANIZATION AND THE INCREASING SUPPLY OF LICIT AND ILLICIT SUBSTANCES
The modern-day increase in supply of alcohol and drugs is a significant contributor to the changing trends in consumption [10▪▪]. The increase in availability and consumption of substances is influenced by numerous factors. Some of these are growing economies, international trade, reduced prices of substances, and access to the internet [11,12]. All of these factors are inherently linked to urbanization and help with understanding the root drivers of harmful substance use in urban environments.
Economic development, international trade and pricing
A study exploring increasing alcohol-related harm in Africa identified economic development and international trade as factors that need to be addressed . The main ways in which economic development influence alcohol consumption are through higher incomes and the ability of corporations to influence policy decisions that favour alcohol consumption. The alcohol industry's penetration in Africa has led to steep competition resulting in increased production, more competitive marketing and pricing schemes . In a qualitative study done in an urbanizing region of the Cape Verde, the cheap price of alcohol (reported as even cheaper than food) and high availability of it were major concerns for community residents . International investment generally makes alcohol more affordable and international trade agreements in Africa have often bypassed domestic laws and national juridical authorities . These factors combined with weak policy infrastructure in some developing countries contribute to increasing patterns of harmful alcohol use.
Marketing, the e-market and ‘legal’ highs
For many, a better internet connection and access to free Wi-Fi is almost synonymous with urban living. Evidently smart cities are connected by the Internet of Things . Although the internet has created many positive connections and conveniences; it has also allowed for connection to the ‘dark web’ and created alternate uncontrollable routes of drug distribution. Tor, the darknet or dark web, is an online network which encrypts communication and allows for online anonymity . This corner of cyberspace motivates sellers and vendors to move away from traditional street-based drug transactions to a more convenient, globalized distribution system. The 2018 world drug report states that drug sales on the darknet may be between 170 and 300 million USD a year and that although trafficking may be limited there are signs of rapid growth [10▪▪]. A US study investigating the geographical patterns of drug sales on Tor confirmed previous United Nations Office on Drugs and Crime findings that New Psychoactive Substances (NPS) and prescription drugs are largely marketed and sold on the darknet .
NPS are synthetic substances created to mimic the effect of illicit drugs. As they fall outside international drug control conventions, they are often referred to as ‘legal highs’. In a sample of 3011 Italian youth, 53.3% had some knowledge of NPS with a higher knowledge in urban areas . NPS use was reported among 4.7% of the sample; however, there were no differences in use between rural and urban areas. In a sample of students in the nonmetropolitan county of Norfolk in the United Kingdom, 8.7% reported use of NPS showing higher prevalence rates than urban areas . A national study of Saudi Arabian schools found that there was a strong association with the heavy use of internet and use of legal and illegal substances [20▪]. These studies suggest links between adolescent internet use, internet-based marketing and sales of potentially dangerous substances. Although not a ‘substance’ addiction, in the context of urbanization, modernization and the internet it is also important to highlight the increase in internet gaming disorders especially in younger population groups.
As a result of the explosive growth in technology and internet usage in the last few years, internet gaming disorder has become a public health concern . The ICD-11 now includes a diagnosis of gaming disorder  and internet gaming disorder was included in the appendix of the updated version of the Diagnostic and Statistical Manual for Mental Disorders . The prevalence of gaming disorders ranges from 0.2 and 8.5% and is highest in Asian countries [23▪]. Gaming disorders are associated with severe negative impact on the physical, psychological, social and occupational functioning of the affected individual and often lead to severe consequences. Currently, there is no direct evidence linking urban living or urban migration with gaming disorder. However, the widespread use of the internet, smartphones and the pathway to substance and behavioural addictions in urbanized areas warrant monitoring. Additionally, prevention strategies and enhancement of protective factors should be explored.
Prescription drug use and the US opioid crisis
The current opioid crisis in the United States has been declared the worst drug overdose crisis in US history. Initially the rise in opioid deaths was matched by an increase in doctors opioid prescriptions which some suggest was higher in urban regions . There have been numerous recent publications analyzing the geographic distributions of opioid abuse and deaths in relation to rural–urban population groups [25▪,26–31]. The results are conflicting. Some report more deaths and higher prescription opioid use in the large metropolitan areas [25▪,31] whereas others state that the rate of death in large metros has decreased with a shift to small and nonmetropolitan areas . There is also evidence pointing to higher rates of harmful use of opioids in rural areas compared to urban [28–31]. This review cannot conclude that the US opioid crisis is driven by factors associated with urbanization. It is, however, worthy to note that high levels of opioid use in urban areas has been linked with increased prescribing because of insufficient knowledge of the harms associated with opioids. Marketing influences from the pharmaceutical industry were also cited as contributing factors [24,32]. A study analyzing data of over 47 000 US adults found that urban adults were more likely to engage in opioid use compared to rural adults because of their higher use of other substances, including alcohol, cannabis and other illicit and prescription drugs. Opioid use in urban residents was also associated with greater use of these substances as children .
URBANIZATION AND THE DEMAND FOR LICIT AND ILLICIT SUBSTANCES
Reviews on urbanization and mental health consistently report that urban migrants experience numerous financial, environmental and cultural stressors [3,5,8,9]. The use of substances as a coping mechanism during periods of stress  or as a form of self-medication for symptoms of depression and anxiety  are well-known psychosocial aetiologies of substance abuse. Thus, it is plausible that the rather merciless fast paced, stressful, lonely and polluted atmosphere of some expanding cities would drive increases in alcohol, tobacco and illicit drug use. However, despite theories postulating the relationship between stress and substance abuse there is limited primary data exploring or confirming causal pathways between specific urban stressors and harmful substance use or substance dependence.
Urban sociocultural stressors and substance use
A three-decade longitudinal study of a minority group in China found that there were significant increases in alcohol abuse, alcohol-related psychosis and alcohol-related liver disease in the last decade [35▪▪]. The major factor influencing this group was modernization and urbanization. With urbanization came changes in patterns of social drinking, increase in divorces and gambling activities. All these factors coupled with a poor recognition of the mental health needs of this minority group contributed to the rise in alcohol use [35▪▪]. In another large-scale study of predominantly urban Chinese nationals, substance use disorder was significantly higher in those living alone . These studies suggest a pathway between urbanization and traditional family structure, cultural norms and risks for increased alcohol consumption. Urbanization has also brought about a breakdown of family structure via migration of parents without their children.
Mass internal urban migration in China and particular Chinese policies have caused many families to be separated. It is estimated that more than 60 million children in China are ‘left behind’ because of their parents seeking work in urban areas . In five Latin American countries, it was found that between 5 and 21% of children had absent parents due transnational urban migration . Lack of parental supervision is repeatedly reported as a significant risk factor for harmful adolescent substance use [39–41]. Thus, although data is currently limited the substance use rates in children left behind because of urban migration should be closely monitored, and prevention strategies should be implemented.
Urban environmental stressors and substance use
In low and middle-income countries, massive slum populations are largely a result of rural urban migration and the inadequacy of cities to cater for migrant groups . Slum populations experience stressors of unemployment, overcrowding and poor sanitation facilities. A study assessing tobacco use in adolescent youth from slum populations in India found that the overall lifetime prevalence of tobacco use was 25%. The prevalence was relatively higher than those reported in nationwide surveys of adolescent tobacco use in India. Tobacco use was higher among those with lower socioeconomic status and adolescents who used tobacco were more likely to use alcohol than those who did not use tobacco. Tobacco use was also significantly associated with family stress, more severe psychological distress and a sense of hopelessness .
Homelessness is another stressor affecting urban migrants. A study in New York assessing homeless people entering an urban hospital emergency department found that patients who were currently homeless had higher rates of past year unhealthy alcohol use, any drug use, heroin use, prescription opioid use and lifetime opioid overdose . It is possible that severe substance dependence contributed to homelessness; however, the studies’ robust finding led the authors to conclude that unstable housing and frank homelessness may be associated with opioid abuse and overdose. This points to infrastructure and economic challenges within urban environments and the complex pathway to substance abuse.
Poor city infrastructure; water, air and noise pollution; insufficient green spaces and traffic congestion are some of the urban environmental factors potentially linked with mental health disorders [3,4,42]. Data specifically linking these urban infrastructure deficits to harmful use of substances is limited. There is, however, evidence of links between environmental factors and general mental health of adolescents. A systematic review on the impact of green space exposure on children and adolescent mental health found that there was a beneficial association between green space exposure and children's emotional and behavioural difficulties . The association was more evident with hyperactivity and inattention problems. It is therefore evident that city planning needs to take into account the mental health of its residents and the need for more green spaces.
Governments should play a more protective role in regulating the supply of harmful licit and illicit addictive substances. Urban planning and infrastructure development should be priority areas for countries experiencing mass urban migration. There should be widespread implementation of evidence-based programmes for adolescent substance use. Many urban centres need to upscale treatment services to meet the needs of rising number of people with harmful substance use and dependence. More longitudinal studies would assist in elucidating the factors that predict harmful substance use after relocation to urban areas and clarify the impact of increasing urban spaces on population's substance consumption trends.
Although limited, there is data reporting significant associations between urbanization processes and increasing supply and demand for licit and illicit substances. Urbanization is associated with many distal physical and social stressors ; however, the pathways to increasing substance use often begin at the level of international trade and policy . Factors ranging from policy, urban infrastructure, internet usage, changes in social and cultural norms, slum environments and frank homelessness may all contribute to rising trends in alcohol, tobacco and drug use in some urban centres. Policymakers need to respond to the impact of rapid unplanned urbanization on mental health and ensure that prevention and treatment services are tailored to changing health needs.
We would like to thank Ms Sherylene Ganesh for her support.
Financial support and sponsorship
N.M.: Recipient of a Cassandra-Miller Butterworth Fellowship for a clinician scientist PhD.
Recipient of a South African National Research Foundation (NRF) Thuthuka Grant.
Funding from the fellowship and grant did not directly support this study.
S.M.: Recipient of a South African Medical Research Council (MRC) self-initiated research Fellowship.
Recipient of a Claude Leon award for early career researchers.
Funding from the fellowship and award did not directly support this study.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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