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Addictions

Four new studies show link between heavy cannabis use, serious health risks

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Cannabis products purchased in Ontario and B.C., including gummies, pre-rolled joints, chocolates and dried flower; April 11, 2025. [Photo credit: Alexandra Keeler]

By Alexandra Keeler

New Canadian research shows a connection between heavy cannabis use and dementia, heart attacks, schizophrenia and even death

Six months ago, doctors in Boston began noticing a concerning trend: young patients were showing up in emergency rooms with atypical symptoms and being diagnosed with heart attacks.

“The link between them was that they were heavy cannabis users,” Dr. Ahmed Mahmoud, a cardiovascular researcher and physician in Boston, told Canadian Affairs in an interview.

These frontline observations mirror emerging evidence by Canadian researchers showing heavy cannabis use is associated with significant adverse health impacts, including heart attacks, schizophrenia and dementia.

Sources warn public health measures are not keeping pace with rapid changes to cannabis products as the market is commercialized.

“The irony of this moment is that society’s risk perception of cannabis is at an all-time low, at the exact moment that the substance is probably having increasingly negative health impacts,” said Dr. Daniel Myran, a physician and Canada Research Chair at the University of Ottawa. Myran was lead researcher on three new Canadian studies on cannabis’ negative health impacts.

Legalization

Canada was the first G7 country to create a commercial cannabis market when it legalized the production and sale of cannabis in 2018.

The drug is now widely used in Canada.

In the 2024 Canadian Cannabis Survey, an annual government survey of cannabis trends, 26 per cent of respondents said they used cannabis for non-medical purposes in the past year, up from 22 per cent in 2018. Among youth, that number was 41 per cent.

Health Canada’s website warns that cannabis use can lower blood pressure and raise heart rates, which can increase the risk of a heart attack. But the warnings on cannabis product labels vary. Some mention risks of anxiety or effects on memory and concentration, but make no mention of cardiovascular risks.

The annual cannabis survey also shows a significant percentage of Canadians remain unaware of cannabis’ health risks.

In the survey, only 70 per cent of respondents said they had enough reliable information to make informed decisions about cannabis use. And 50 per cent of respondents said they had not seen any education campaigns or public health messages about cannabis.

At the same time, researchers are finding mounting evidence that cannabis use is associated with health risks.

A 2023 study by researchers at the University of Calgary, the University of Alberta and Alberta Health Services found that adults with cannabis use disorder faced a 60 per cent higher risk of experiencing adverse cardiovascular events — including heart attacks. Cannabis use disorder is marked by the inability to stop using cannabis despite negative consequences, such as work, social, legal or health issues.

Between February and April of this year, three other Canadian studies linked frequent cannabis use to elevated risks of developing schizophrenia, dementia and mortality. These studies were primarily conducted by researchers at the Ottawa Hospital Research Institute and ICES uOttawa (formerly the Institute for Clinical Evaluative Sciences).

“These results suggest that individuals who require hospital-based care for a [cannabis use disorder] may be at increased risk of premature death,” said the study linking cannabis-related hospital visits with increased mortality rates.

The three 2024 studies all examined the impacts of severe cannabis use, suggesting more moderate users may face lower risks. The researchers also cautioned that their research shows a correlation between heavy cannabis use and adverse health effects, but does not establish causality.

 

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Budtenders

Health experts say they are troubled by the widespread perception that cannabis is entirely benign.

“It has some benefits, it has some side effects,” said Mahmoud, the Boston cardiovascular researcher. “We need to raise awareness about the side effects as well as the benefits.”

Some also expressed concern that the commercialization of cannabis products in Canada has created a race to produce products with elevated levels of THC, the main psychoactive compound that produces a “high.”

THC levels have more than doubled since legalization, yet even products with high THC levels are marketed as harmless.

“The products that are on the market are evolving in ways that are concerning,” Myran said. “Higher THC products are associated with considerably more risk.”

Myran views cannabis decriminalization as a public health success, because it keeps young people out of the criminal justice system and reduces inequities faced by Indigenous and racialized groups.

“[But] I do not think that you need to create a commercial cannabis market or industry in order to achieve those public health benefits,” he said.

Since decriminalization, the provinces have taken different approaches to regulating cannabis. But even in provinces where governments control cannabis distribution, such as New Brunswick and Nova Scotia, products with high THC levels dominate retail shelves and online storefronts.

In Myran’s view, federal and provincial governments should instead be focused on curbing harmful use patterns, rather than promoting cannabis sales.

Ian Culbert, executive director of the Canadian Public Health Association, thinks governments’ financial interest in the cannabis industry creates a conflict of interest.

“[As with] all regulated substances, governments are addicted to the revenue they create,” he said. “But they also have a responsibility to safeguard the well-being of citizens.”

Culbert believes cannabis retailers should be required to educate customers about health risks — just as bartenders are required to undergo Smart Serve training and lottery corporations are required to mitigate risks of gambling addiction.

“Give ‘budtenders’ the training around potential health risks,” he said.

“While cannabis may not be the cause of some of these negative health events … it is the intersection at which an intervention can take place through the transaction of sales. So is there something we can do there that can change the trajectory of a person’s life?”


This article was produced through the Breaking Needles Fellowship Program, which provided a grant to Canadian Affairs, a digital media outlet, to fund journalism exploring addiction and crime in Canada. Articles produced through the Fellowship are co-published by Break The Needle and Canadian Affairs.

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2025 Federal Election

Study links B.C.’s drug policies to more overdoses, but researchers urge caution

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By Alexandra Keeler

A study links B.C.’s safer supply and decriminalization to more opioid hospitalizations, but experts note its limitations

A new study says B.C.’s safer supply and decriminalization policies may have failed to reduce overdoses. Furthermore, the very policies designed to help drug users may have actually increased hospitalizations.

“Neither the safer opioid supply policy nor the decriminalization of drug possession appeared to mitigate the opioid crisis, and both were associated with an increase in opioid overdose hospitalizations,” the study says.

The study has sparked debate, with some pointing to it as proof that B.C.’s drug policies failed. Others have questioned the study’s methodology and conclusions.

“The question we want to know the answer to [but cannot] is how many opioid hospitalizations would have occurred had the policy not have been implemented,” said Michael Wallace, a biostatistician and associate professor at the University of Waterloo.

“We can never come up with truly definitive conclusions in cases such as this, no matter what data we have, short of being able to magically duplicate B.C.”

Jumping to conclusions

B.C.’s controversial safer supply policies provide drug users with prescription opioids as an alternative to toxic street drugs. Its decriminalization policy permitted drug users to possess otherwise illegal substances for personal use.

The peer-reviewed study was led by health economist Hai Nguyen and conducted by researchers from Memorial University in Newfoundland, the University of Manitoba and Weill Cornell Medicine, a medical school in New York City. It was published in the medical journal JAMA Health Forum on March 21.

The researchers used a statistical method to create a “synthetic” comparison group, since there is no ideal control group. The researchers then compared B.C. to other provinces to assess the impact of certain drug policies.

Examining data from 2016 to 2023, the study links B.C.’s safer supply policies to a 33 per cent rise in opioid hospitalizations.

The study says the province’s decriminalization policies further drove up hospitalizations by 58 per cent.

“Neither the safer supply policy nor the subsequent decriminalization of drug possession appeared to alleviate the opioid crisis,” the study concludes. “Instead, both were associated with an increase in opioid overdose hospitalizations.”

The B.C. government rolled back decriminalization in April 2024 in response to widespread concerns over public drug use. This February, the province also officially acknowledged that diversion of safer supply drugs does occur.

The study did not conclusively determine whether the increase in hospital visits was due to diverted safer supply opioids, the toxic illicit supply, or other factors.

“There was insufficient evidence to conclusively attribute an increase in opioid overdose deaths to these policy changes,” the study says.

Nguyen’s team had published an earlier, 2024 study in JAMA Internal Medicine that also linked safer supply to increased hospitalizations. However, it failed to control for key confounders such as employment rates and naloxone access. Their 2025 study better accounts for these variables using the synthetic comparison group method.

The study’s authors did not respond to Canadian Affairs’ requests for comment.

 

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Correlation vs. causation

Chris Perlman, a health data and addiction expert at the University of Waterloo, says more studies are needed.

He believes the findings are weak, as they show correlation but not causation.

“The study provides a small signal that the rates of hospitalization have changed, but I wouldn’t conclude that it can be solely attributed to the safer supply and decrim[inalization] policy decisions,” said Perlman.

He also noted the rise in hospitalizations doesn’t necessarily mean more overdoses. Rather, more people may be reaching hospitals in time for treatment.

“Given that the [overdose] rate may have gone down, I wonder if we’re simply seeing an effect where more persons survive an overdose and actually receive treatment in hospital where they would have died in the pre-policy time period,” he said.

The Nguyen study acknowledges this possibility.

“The observed increase in opioid hospitalizations, without a corresponding increase in opioid deaths, may reflect greater willingness to seek medical assistance because decriminalization could reduce the stigma associated with drug use,” it says.

“However, it is also possible that reduced stigma and removal of criminal penalties facilitated the diversion of safer opioids, contributing to increased hospitalizations.”

Karen Urbanoski, an associate professor in the Public Health and Social Policy department at the University of Victoria, is more critical.

“The [study’s] findings do not warrant the conclusion that these policies are causally associated with increased hospitalization or overdose,” said Urbanoski, who also holds the Canada Research Chair in Substance Use, Addictions and Health Services.

Her team published a study in November 2023 that measured safer supply’s impact on mortality and acute care visits. It found safer supply opioids did reduce overdose deaths.

Critics, however, raised concerns that her study misrepresented its underlying data and showed no statistically significant reduction in deaths after accounting for confounding factors.

The Nguyen study differs from Urbanoski’s. While Urbanoski’s team focused on individual-level outcomes, the Nguyen study analyzed broader, population-level effects, including diversion.

Wallace, the biostatistician, agrees more individual-level data could strengthen analysis, but does not believe it undermines the study’s conclusions. Wallace thinks the researchers did their best with the available data they had.

“We do not have a ‘copy’ of B.C. where the policies weren’t implemented to compare with,” said Wallace.

B.C.’s overdose rate of 775 per 100,000 is well above the national average of 533.

Elenore Sturko, a Conservative MLA for Surrey-Cloverdale, has been a vocal critic of B.C.’s decriminalization and safer supply policies.

“If the government doesn’t want to believe this study, well then I invite them to do a similar study,” she told reporters on March 27.

“Show us the evidence that they have failed to show us since 2020,” she added, referring to the year B.C. implemented safer supply.


This article was produced through the Breaking Needles Fellowship Program, which provided a grant to Canadian Affairs, a digital media outlet, to fund journalism exploring addiction and crime in Canada. Articles produced through the Fellowship are co-published by Break The Needle and Canadian Affairs.

Our content is always free – but if you want to help us commission more high-quality journalism,

consider getting a voluntary paid subscription.

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Addictions

Addiction experts demand witnessed dosing guidelines after pharmacy scam exposed

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By Alexandra Keeler 

The move follows explosive revelations that more than 60 B.C. pharmacies were allegedly participating in a scheme to overbill the government under its safer supply program. The scheme involved pharmacies incentivizing clients to fill prescriptions they did not require by offering them cash or rewards. Some of those clients then sold the drugs on the black market.

An addiction medicine advocacy group is urging B.C. to promptly issue new guidelines for witnessed dosing of drugs dispensed under the province’s controversial safer supply program.

In a March 24 letter to B.C.’s health minister, Addiction Medicine Canada criticized the BC Centre on Substance Use for dragging its feet on delivering the guidelines and downplaying the harms of prescription opioids.

The centre, a government-funded research hub, was tasked by the B.C. government with developing the guidelines after B.C. pledged in February to return to witnessed dosing. The government’s promise followed revelations that many B.C. pharmacies were exploiting rules permitting patients to take safer supply opioids home with them, leading to abuse of the program.

“I think this is just a delay,” said Dr. Jenny Melamed, a Surrey-based family physician and addiction specialist who signed the Addiction Medicine Canada letter. But she urged the centre to act promptly to release new guidelines.

“We’re doing harm and we cannot just leave people where they are.”

Addiction Medicine Canada’s letter also includes recommendations for moving clients off addictive opioids altogether.

“We should go back to evidence-based medicine, where we have medications that work for people in addiction,” said Melamed.

‘Best for patients’

On Feb. 19, the B.C. government said it would return to a witnessed dosing model. This model — which had been in place prior to the pandemic — will require safer supply participants to take prescribed opioids under the supervision of health-care professionals.

The move follows explosive revelations that more than 60 B.C. pharmacies were allegedly participating in a scheme to overbill the government under its safer supply program. The scheme involved pharmacies incentivizing clients to fill prescriptions they did not require by offering them cash or rewards. Some of those clients then sold the drugs on the black market.

In its Feb. 19 announcement, the province said new participants in the safer supply program would immediately be subject to the witnessed dosing requirement. For existing clients of the program, new guidelines would be forthcoming.

“The Ministry will work with the BC Centre on Substance Use to rapidly develop clinical guidelines to support prescribers that also takes into account what’s best for patients and their safety,” Kendra Wong, a spokesperson for B.C.’s health ministry, told Canadian Affairs in an emailed statement on Feb. 27.

More than a month later, addiction specialists are still waiting.

 

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According to Addiction Medicine Canada’s letter, the BC Centre on Substance Use posed “fundamental questions” to the B.C. government, potentially causing the delay.

“We’re stuck in a place where the government publicly has said it’s told BCCSU to make guidance, and BCCSU has said it’s waiting for government to tell them what to do,” Melamed told Canadian Affairs.

This lag has frustrated addiction specialists, who argue the lack of clear guidance is impeding the transition to witnessed dosing and jeopardizing patient care. They warn that permitting take-home drugs leads to more diversion onto the streets, putting individuals at greater risk.

“Diversion of prescribed alternatives expands the number of people using opioids, and dying from hydromorphone and fentanyl use,” reads the letter, which was also co-signed by Dr. Robert Cooper and Dr. Michael Lester. The doctors are founding board members of Addiction Medicine Canada, a nonprofit that advises on addiction medicine and advocates for research-based treatment options.

“We have had people come in [to our clinic] and say they’ve accessed hydromorphone on the street and now they would like us to continue [prescribing] it,” Melamed told Canadian Affairs.

A spokesperson for the BC Centre on Substance Use declined to comment, referring Canadian Affairs to the Ministry of Health. The ministry was unable to provide comment by the publication deadline.

Big challenges

Under the witnessed dosing model, doctors, nurses and pharmacists will oversee consumption of opioids such as hydromorphone, methadone and morphine in clinics or pharmacies.

The shift back to witnessed dosing will place significant demands on pharmacists and patients. In April 2024, an estimated 4,400 people participated in B.C.’s safer supply program.

Chris Chiew, vice president of pharmacy and health-care innovation at the pharmacy chain London Drugs, told Canadian Affairs that the chain’s pharmacists will supervise consumption in semi-private booths.

Nathan Wong, a B.C.-based pharmacist who left the profession in 2024, fears witnessed dosing will overwhelm already overburdened pharmacists, creating new barriers to care.

“One of the biggest challenges of the retail pharmacy model is that there is a tension between making commercial profit, and being able to spend the necessary time with the patient to do a good and thorough job,” he said.

“Pharmacists often feel rushed to check prescriptions, and may not have the time to perform detailed patient counselling.”

Others say the return to witnessed dosing could create serious challenges for individuals who do not live close to health-care providers.

Shelley Singer, a resident of Cowichan Bay, B.C., on Vancouver Island, says it was difficult to make multiple, daily visits to a pharmacy each day when her daughter was placed on witnessed dosing years ago.

“It was ridiculous,” said Singer, whose local pharmacy is a 15-minute drive from her home. As a retiree, she was able to drive her daughter to the pharmacy twice a day for her doses. But she worries about patients who do not have that kind of support.

“I don’t believe witnessed supply is the way to go,” said Singer, who credits safer supply with saving her daughter’s life.

Melamed notes that not all safer supply medications require witnessed dosing.

“Methadone is under witness dosing because you start low and go slow, and then it’s based on a contingency management program,” she said. “When the urine shows evidence of no other drug, when the person is stable, [they can] take it at home.”

She also noted that Suboxone, a daily medication that prevents opioid highs, reduces cravings and alleviates withdrawal, does not require strict supervision.

Kendra Wong, of the B.C. health ministry, told Canadian Affairs that long-acting medications such as methadone and buprenorphine could be reintroduced to help reduce the strain on health-care professionals and patients.

“There are medications available through the [safer supply] program that have to be taken less often than others — some as far apart as every two to three days,” said Wong.

“Clinicians may choose to transition patients to those medications so that they have to come in less regularly.”

Such an approach would align with Addiction Medicine Canada’s recommendations to the ministry.

The group says it supports supervised dosing of hydromorphone as a short-term solution to prevent diversion. But Melamed said the long-term goal of any addiction treatment program should be to reduce users’ reliance on opioids.

The group recommends combining safer supply hydromorphone with opioid agonist therapies. These therapies use controlled medications to reduce withdrawal symptoms, cravings and some of the risks associated with addiction.

They also recommend limiting unsupervised hydromorphone to a maximum of five 8 mg tablets a day — down from the 30 tablets currently permitted with take-home supplies. And they recommend that doses be tapered over time.

“This protocol is being used with success by clinicians in B.C. and elsewhere,” the letter says.

“Please ensure that the administrative delay of the implementation of your new policy is not used to continue to harm the public.”


This article was produced through the Breaking Needles Fellowship Program, which provided a grant to Canadian Affairs, a digital media outlet, to fund journalism exploring addiction and crime in Canada. Articles produced through the Fellowship are co-published by Break The Needle and Canadian Affairs.


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