Last updated May 2014
John Howard and Jan Copeland, NCPIC
Cannabis is the most commonly used illicit drug in the Western world.1 Despite the extent of its use, the effects of cannabis use and dependence on health and psycho-social functioning are often under-recognised. This represents missed opportunities for health promotion and early interventions for young people who may already be experiencing cannabis use-related difficulties.
Overall, use of cannabis presents a mixed picture, with use stabilising in some countries, yet reducing or increasing in others. Also evident, is that gender differences appear to be diminishing among young people who use cannabis, and Indigenous communities in the US, Australia and some Pacific Islands appear to have higher levels of cannabis use than the non-Indigenous populations.2
In addition, there is debate regarding the regulation or decriminalisation of cannabis. High profile sports people, entertainers and even politicians have made their views and histories of cannabis use known, or have been exposed by mainstream or web-based social media. Young people are aware of this, and despite exposure to confusing messages, are expected to make informed choices. Also relevant is past and present use of cannabis by some parents, older siblings and even teachers, reinforcing a view that cannabis is a soft drug with minimal consequences for physical and mental health. Such views ignore significant changes in the patterns of cannabis use over the past 40 years, which include a decline in the age of initiation of use, an increase in use of more potent cannabis, and more frequent and heavier use by young people with peers and when alone. 2, 3
Potential harms associated with cannabis use
As most drug use behaviours are initiated during adolescence, it is timely to explore what is known of the use of cannabis among young people aged around 15 to 16 years. There is increasing concern about the impact of early onset, regular and heavy cannabis use on the psycho-social development of young people. 3 Evidence is available identifying cognitive impairment, in addition to obvious risks for respiratory disease, such as chronic bronchitis. In addition, many smokers mix cannabis with tobacco and are regular tobacco smokers. There is evidence that some of the negative respiratory effects of cannabis and tobacco may be additive. 3 There are also impacts on educational performance and attainment, with robust findings linking early onset of cannabis use and poorer educational achievement.4
Frequent and heavy use of cannabis exacerbates underlying mental health conditions, including schizophrenia, manifesting as increased symptoms and severity, non-compliance with treatment and more frequent hospitalisations. Recent studies indicate there is a greater risk for schizophrenia in vulnerable people who use cannabis more than three times per week, prior to the age of 15 years.5-8
What then is known of the prevalence of cannabis use among young people aged around 15 to 16 years? Data are available from a variety of sources on cannabis use for this age group in 96 countries.9-20
The majority of data come from three survey instruments, which are administered in groups in selected schools. Sample sizes vary, but attempts are made to ensure that the students sampled are representative of the student populations of their respective countries. Thus, there is an attempt to ensure the specific rural and urban, gender and ethnic mix is representative:
the WHO s Global Student Health Survey (GSHS)10, used in many countries around the world;
the WHO s Health Behaviour in School-aged Children (HBSC)11 survey, used mainly in Europe; and
the Youth Risk Behaviour Survey (YRBS)12 developed by the Centres for Disease Control and Prevention, is used in the USA and territories, and adapted in some other countries, for example Japan. Other countries have used their own survey instruments; for example, Australia, Brazil, Cambodia, Chile, Colombia, Hong Kong, Israel, Mexico, New Zealand, and South Africa. The remaining South and Central American counties that have utilised a survey instrument similar to the YRBS: Argentina, Bahamas, Belize, Brazil, Dominica, Dominican Republic, Ecuador, El Salvador, Grenada, Guatemala, Guyana, Haiti, Honduras, Jamaica, Panama, Paraguay, Saint Kitts & Nevis, Saint Lucia and Venezuela.
While the data come from a variety of surveys, the core questions remain essentially similar: During your life, how many times have you used marijuana? (with various other names for cannabis also mentioned as needed) See below Table 1.
Table 1: Survey question examples
GSHS and HBSC
During the past 30 days, how many times have you used marijuana?
During your life, how many times have you used marijuana?
Australian school survey
How many times, if ever, have you smoked or used marijuana/cannabis
However, not all countries using the same survey instrument include all questions; this is especially the case for those using the WHO s GSHS.This appears to be due to particular religious or cultural sensitivities regarding use of illegal substances. In some countries not even have you ever used cannabis? is included, and in others, where ever used was included, recent use was not. Unfortunately, such omissions can lead to some potential lack of clarity when addressing public health concerns. As a consequence, meaningful comparisons of prevalence between countries for ever use and for use in the last month are only possible when asked and the same survey is used. Also, survey years vary and thus, some comparisons require caution.
In addition, while students across many grades of secondary schools are surveyed, the data here mainly focus on those aged around 15 years.
Unfortunately, there are significant data gaps, primarily from African nations and South and East Asia, where there are very youthful populations, as well as developing, emerging or transitional economies. Additional issues relate to a number of nations in these regions being on drug transhipment routes, or producers or manufacturers of illicit substances.
Countries that have recently used the WHO GSHS but omitted cannabis questions or did not report cannabis data: Anguilla, Benin, Botswana, Cayman Islands, China, Djibouti, Egypt, Fiji, FR Macedonia, India, Indonesia, Iraq, Jordan, Kenya, Libya, Malawi, Maldives, Montserrat, Myanmar, Namibia, Nauru, Nicaragua, Niue, Oman, Pakistan, Philippines, Qatar, Senegal, Seychelles, Sri Lanka, Suriname, Syria, Tajikistan, Tanzania, Thailand, Tunisia, UAE, Uganda, Yemen, Zambia and Zimbabwe.
Countries that have recently used the WHO HBSC but omitted cannabis questions or did not report cannabis data: Sweden and Turkey.
Countries that have completed other youth surveys, but omitted cannabis questions or did not report cannabis data: Afghanistan, Bangladesh, Cambodia, Iran, Lao PDR, Nepal, Papua and New Guinea, Serbia, Singapore, South Korea and Viet Nam.