The Effects of Cannabis on Health - What the Current Research Says

Depending on who you ask you may be told many different and conflicting things about cannabis. Really, the debate should be driven by careful scientific research. It can make you feel more relaxed and has reported, sometimes proven, medical use. However, it can also lead to acute attacks of paranoia and anxiety, could be addictive and it is feared that it causes schizophrenia. What often isn’t explained is how these conclusions were reached, nor the extent to which concrete conclusions can be extrapolated from the data. The intent of this article is to give a more detailed understanding of the science behind some key claims.

What’s the harm?

  In 2007, the Advisory Council on Misuse of Drugs (ACMD) published a paper that sought to make an unbiased assessment of the relative harms of various commonly misused substances. Each substance was assigned a “harm score” out of nine. Three points were available for each perceived harm associated with its abuse: “Physical Harm”, “Induction of Dependence” and “Social Harm”. Groups of independent experts from different relevant disciplines carried out the rating. Cannabis scored 4 out of a possible 9 points, lower than tobacco at 4.87 and alcohol at 5.54 [1]. The study notes that the legality and availability of cigarettes and alcohol preclude direct comparison with other substances; however their prevalence and extensive use provide a familiar benchmark for some degree of comparison to be made.

  Census data and alcohol and cannabis use and death rates are similarly revealing. [2–5]. Preliminary calculations show 35.7 deaths per 100,000 alcohol users are attributed to alcohol consumption, compared to just 0.31 per 100,000 for cannabis users. If the cause of death is attributed to solely cannabis, rather than in combination with other drugs, that number falls to 0.04 deaths per 100, 000 users.

  This is because only one or two deaths a year are ever attributed to cannabis use, out of an estimated 2, 238, 704 regular users in England and Wales. Of course, mortality rates aren’t a measure of total harm by them selves, but they do indicate a dramatic difference. Whilst these are approximate figures, the deviation of population and usage statistics from actual levels is likely only marginal, and the lower proportion of the population regularly using cannabis does not prevent effective comparison.

The power of addiction

  There is a consensus that cannabis is addictive, but the question is one of degrees - heroin and cigarettes are both addictive, but the former is clearly worse.

  So how do you measure addictiveness? In the ACMD study mentioned above, one assessment category was called ‘Induction of Dependence’. Based on the subjective ratings by the independent expert groups, cannabis scored an average of 1.51 out of a possible 3 in this category. For comparison, alcohol scored 1.93 and tobacco scored 2.21.

  Through the subjective ratings collected, this study suggests that cannabis is less addictive than alcohol and much less addictive than tobacco.

  However, due to the complexity of addiction, it remains a difficult phenomenon to measure objectively. A qualitative assessment of addiction has been attempted through comparing the severity of withdrawal from cannabis to that from tobacco. Regular users abstaining from one or the other for 5 days reported similar levels of irritation, anxiety, low mood, restlessness and disturbed sleep [6]. The major limitation of this study is that it was not ‘blinded’, meaning participants knew which substance they were withdrawing from, potentially influencing their answers in the self-report questionnaires used. How major the ‘blinding’ issue is remains a point of debate.

Medical benefit

  Medicinal marijuana sounds strange. Cannabis is illegal, and it must be illegal because it is harmful, yet it’s being used to treat disease. Cannabis has in fact shown much promise in reducing some of the more distressing symptoms resulting from multiple sclerosis, HIV/AIDS, chemotherapy and treating chronic neuropathic pain.

  In Germany, cannabis extract (not cigarettes but the two active ingredients purified) has been granted a license until 2015 as a treatment for the spasticity that arises as a consequence of multiple sclerosis [7]. For a video showing just how effective cannabis use can be, watch Jacqueline Patterson’s account on the Talking Drugs website.

  There is also a lot of positive evidence demonstrating that cannabis cigarettes and cannabis-derived drugs (such as dronabinol) are effective treatments for appetite loss, nausea and vomiting arising from chemotherapy of cancer and HIV/AIDS. However, these studies sometimes show that cannabis is only as effective as current treatments [7]. Similarly, although cannabis seems to have little affect on acute pain, it has been shown to result in a significant reduction in perception of chronic neuropathic pain. This is a constant pain reported by a patient but with no discernible cause. The trials producing these conclusions were placebo controlled, and the treatment group consistently reported around 30% improvement in pain relief over the placebo group.

The psychosis risk

  The most publicised danger of cannabis is the clear and established link between cannabis use in adolescence and adult ‘schizophreniform’ disorders.

  An analysis of five studies found that the overall chances of developing a schizophreniform disorder after use of cannabis is double that for non-users [8]. This broad, all-encompassing conclusion is slightly misleading, however, as it ignores the fact that the heaviest users, typically defined as those that have used cannabis more than 50 times by the age 18-20, face the most significant risk. One study strongly indicated that in fact only use of cannabis (3+ times) before the age of 15 increases risk of schizophrenia [9]. It is also important to recognize that ‘schizophreniform’ does not mean schizophrenia per se, but encompasses a much broader range of psychological disorders.

  Furthermore, it is becoming clear that use of cannabis is only one of a wide variety of influences that can increase risk of schizophrenia in vulnerable individuals. It is neither necessary nor sufficient to cause the disorder.

  Use of cannabis is not always as black and white as it is portrayed to be. It is clearly a complex issue requiring a mature well-informed debate to ensure a fair and beneficial fate is decided for its control as a substance.

Recommended reading: - cannabis information sheet

The website of the Independent Scientific Committee on Drugs (ISCD) and their information page about cannabis. It is plainly, honestly written and very thorough.



[1] D. Nutt, L. A. King, W. Saulsbury, and C. Blakemore, “Development of a rational scale to assess the harm of drugs of potential misuse,” Lancet, pp. 1047–1053, 2007.

[2] D. of H. Home Office, “Safe. Sensible. Social. The next steps in the National Alcohol Strategy,” 2007.

[3] ONS, “Statistical Bulletin Alcohol-related Deaths in the UK,” 2010.

[4] S. Dunstan and ONS, “General Lifestyle Survey Overview,” 2010.

[5] UNODC, “World Drug Report 2012,” 2012.

[6] R. Vandrey, A. Budney, J. Hughes, and A. Liguori, “A Within-Subject Comparison of Withdrawal Symptoms During Abstinence from Cannabis, Tobacco and Both Substances,” Drug and Alchohol Dependence, vol. 92, pp. 48–54, 2008.

[7] F. Grotenhermen and K. Müller-Vahl, “The therapeutic potential of cannabis and cannabinoids.,” Deutsches Ärzteblatt international, vol. 109, no. 29–30, pp. 495–501, Jul. 2012.

[8] L. Arseneault, “Causal association between cannabis and psychosis: examination of the evidence,” The British Journal of Psychiatry, vol. 184, no. 2, pp. 110–117, Feb. 2004.

[9] L. Arseneault, M. Cannon, R. Poulton, R. Murray, A. Caspi, and T. E. Moffitt, “Cannabis use in adolescence and risk for adult psychosis : longitudinal prospective study Drug points,” Bmj, vol. 325, no. November, pp. 1212–1213, 2002.