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  • People who inject opioids are at increased risk for overdose and other harms
  • Observational studies suggest that people who use cannabis daily inject opioids less frequently
  • Controlled studies are needed to better understand the potential of cannabis as harm reduction

In general, people who share equipment for injecting street drugs are at heightened risk of harm, including exposure to blood-borne germs (such as various harmful bacteria) and viruses such as HIV and hepatitis B and C. Injecting street drugs is also associated with an increased risk of overdose. In recent years, the supply of street drugs has become contaminated with the very powerful opioid fentanyl and its analogues. As a result, the risk of accidental overdose and death arising from the use of street drugs has soared, leaving communities devastated.

A team of scientists at the British Columbia Centre on Substance Use and universities in that province have been conducting observational studies analyzing the association between self-reported cannabis use and injection of street drugs. Cannabis contains many compounds, called cannabinoids, that can affect nerves and the brain. The researchers found that people who used cannabis daily were less likely to inject opioids daily.

This finding, the researchers stated, adds to an accumulating body of research that supports the idea of conducting studies “to test whether controlled administration of cannabinoids impacts the frequency of opioid injection among people who inject drugs.”

Study details

Researchers collected and analyzed health and behavioural information from three ongoing studies called Access, Arys and Vidus. These studies included a mix of HIV-positive and HIV-negative people, as well as some young people between the ages of 14 and 26. All participants were recruited from neighborhoods in Vancouver and used a range of substances.

At the start of the study and every six months thereafter, participants answered surveys and provided blood samples for testing for HIV and/or hepatitis C virus (HCV). According to the study researchers, a nurse provided “basic medical care and referrals to health and social services if requested.”

Researchers focused on data collected between 2005 and 2018 from 2,619 adults (most were between 26 and 46 years), 65% were men and 35% were women.


At their first study visit, 28% of participants reported daily cannabis use and 46% reported daily injection drug use. According to the researchers, the main substances injected were as follows:

  • heroin – 31%
  • cocaine – 7%
  • crack cocaine – 1%
  • “speedball” (heroin + cocaine) – 3%
  • meth (methamphetamine) – 12%
  • “goofball” (heroin + meth) – 2%
  • prescription opioids – 6%

At the start of the study, researchers found a statistical association between daily use of cannabis (vs. non-daily use) and a reduced risk for daily injecting. When researchers zeroed in on the substances injected, they found that daily cannabis use was associated only with a reduced risk for injecting opioids, not stimulants.

Changes in time

Over the course of the study, the researchers found that people who used daily cannabis were 17% less likely to inject opioids daily. That is, the effect of daily cannabis on a reduced risk of injecting was seen throughout the study. Furthermore, the overall trend in the study in people who were not using daily cannabis was toward more frequent (daily) injecting of opioids.

Bear in mind

According to the researchers, their findings “contribute to the preliminary, preclinical, clinical and ecological evidence describing beneficial associations with cannabis use for people at risk for drug-related harms, including people who inject drugs.”

The present study was observational in design, so it cannot prove that daily use of cannabis caused a reduction in injection of opioids.

In the past several years, different observational studies have found the following:

  • Frequent use of cannabis was associated with a reduced likelihood of initiating injection of drugs among people who used substances.
  • Analysis of urine samples has found that people who used cannabis recently were less likely to be exposed to fentanyl compared to urine samples from people who had not used cannabis recently.

Effects on policy

The researchers stated, “Our findings that cannabis use was associated with a decreased frequency of injection opioid use provides evidence to inform policy surrounding the harm reduction application of cannabis for people who inject drugs.”

A possible research agenda

By including people who use drugs in discussion and planning, researchers can develop a research agenda that can meet the needs of people who are ready to reduce their dependence on injecting opioids.

The researchers stated that “there is a need to evaluate [cannabis-associated] harm reduction–based interventions and to conduct clinical trials to identify causal relationships, if any, between controlled administration of cannabinoids and overdose risk.”

It is possible that frequent use of cannabis may cause other problems or make some underlying health conditions worse. As an example, research has found that some people with psychosis who use cannabis can develop more severe symptoms of psychosis and require hospitalization than people who do not have psychosis who use cannabis. Therefore, it is important that future research with cannabis assess its potential benefits and risks.

In addition to a potential for harm reduction with opioid injection, cannabis may have other applications. The B.C. team notes that past research has found that people who use cannabis daily were “more likely to report use of cannabis to address pain, nausea and [HIV-related issues]…”

For the future

The present study is an important step forward in cannabis research. But it will take many years to design a potential study, compete for scarce research funds and, finally, if funding occurs, implement studies to assess the clinical impact of cannabis (or cannabinoids) on the health of people who use drugs. Such studies will need to explore at least the following:

  • Which cannabinoid or combination of cannabinoids is best to help reduce frequent injecting of opioids (and thereby reduce the risk of overdose)?
  • How should cannabinoids be consumed—orally, inhalation or by other means—to be most effective?
  • Are different doses of cannabinoids needed depending on age, ethno-racial group, gender, or weight?
  • Do cannabinoids interact with different medicines used by people with chronic health conditions?

—Sean R. Hosein


British Columbia Centre on Substance Use

Lessons Not Learned: The Overdose Crisis in Canada – The Positive Side

Why many people may not disclose substance use to healthcare providers – CATIE News

Estimated number of people who inject drugs and coverage of harm reduction programs in Canada – CATIE News

Indigenous harm reduction = Reducing the harms of colonialism ­– Canadian Aboriginal AIDS Network (CAAN), Interagency Coalition on AIDS and Development (ICAD)

Indigenous-centred approaches to harm reduction and hepatitis C programs – CATIE

British Columbia study finds higher levels of age-related issues in HIV-positive people – CATIE News

Assessing the cascade of care for opioid use disorder in British Columbia – CATIE News


  1. Reddon H, DeBeck K, Socias ME, et al. Frequent cannabis use is negatively associated with frequency of injection drug use among people who inject drugs in a Canadian setting. Cannabis and Cannabinoid Research. 2021; in press.
  2. Reddon H, DeBeck K, Socias ME, et al. Cannabis use is associated with lower rates of initiation of injection drug use among street-involved youth: A longitudinal analysis. Drug and Alcohol Review. 2018 Mar;37(3):421-428.
  3. Coronado-Montoya S, Morissette F, Abdel-Baki A, et al. Preventive interventions targeting cannabis use and related harms in people with psychosis: A systematic review. Early Intervention in Psychiatry. 2021; in press.
  4. Goodman S, Fischer B, Hammond D. Lower-risk cannabis use guidelines: Adherence in Canada and the U.S. American Journal of Preventive Medicine. 2020 Dec;59(6):e211-e220.