There exists a long list of reasons not to take drugs: addiction, decreased productivity, deleterious health effects, damage to personal relationships, and the chance of fatal overdose are just a handful. To this tally of negative consequences, researchers have added another: an increased risk of suicide.
A team of Iranian epidemiologists searched the literature for papers on substance use disorder (SUD) and suicide. Because the researchers were primarily concerned with drugs other than alcohol, they excluded papers that did not separate the effects of alcohol from other drugs. They also excluded data on accidental overdose deaths. After applying these and other criteria, the team selected 43 papers (which represented a total of 870,967 participants) and performed a meta-analysis (i.e., a data intensive review).
Compared to people who did not take drugs, the team found that people who took drugs were twice as likely to think about suicide, 2.5 times as likely to attempt suicide, and 1.5 times as likely (i.e., 50% more likely) to die from suicide*. The data on the latter, which came from seven papers, is shown below:
The authors were not able to determine if specific kinds of drugs were linked to suicide. A follow-up study looking specifically at the effect of marijuana and opioids on suicide would be of considerable public interest given the trend in legalization and the current painkiller epidemic, respectively, in the U.S.
Similarly, it is not possible from this analysis to determine if substance abuse is a cause of suicide. People who take drugs may do so out of boredom, pain, or depression, or they may even be attempting to treat a mental illness through self-medication. In such instances, drug abuse would be symptomatic of an underlying issue and not necessarily the cause of suicide.
Source: Jalal Poorolajal, Tahereh Haghtalab, Mehran Farhadi, Nahid Darvishi. "Substance use disorder and risk of suicidal ideation, suicide attempt and suicide death: a meta-analysis." J Public Health 38 (3): e282-e291. Published online: 26-Oct-2015. doi: 10.1093/pubmed/fdv148.
*Because the 95% confidence interval overlaps with 1.0, the results are not statistically significant. However, that does not necessarily mean the data is inconclusive. Since drug abuse is linked to suicidal thinking and suicide attempts, it is reasonable to conclude that it is also linked to deaths by suicide.