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Drug users are seen in a Downtown Eastside alley in Vancouver in 2016.

<252>Rafal Gerszak/<252>The Globe and Mail

A study of drug use in Vancouver's Downtown Eastside concluded with 100 per cent of participants who used illicit opioids testing positive for fentanyl, raising concerns that higher opioid tolerance from the powerful synthetic drug could threaten the effectiveness of substitution treatment.

The five-month study, led by University of British Columbia psychiatry professor William Honer, involved 237 high-risk participants. Of those, about half used opioids, either prescribed (such as methadone and buprenorphine) or non-prescribed (such as illicit heroin). Severe mental-health issues also played a significant role: About half had psychosis and one-third had mood disorders, illnesses which increase the likelihood of using illicit drugs.

When the study began in March, 2017, about 45 per cent of urine samples from participants who used non-prescribed opioids tested positive for fentanyl. By June-July, the figure had climbed to 100 per cent.

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The initial climb in March-April coincided with a spike in overdose calls and deaths in Vancouver. But when the number of those calls and deaths began to go down toward the end of the five-month study period, fentanyl-positive tests continued to climb. Dr. Honer, who is also a senior researcher with the B.C. Mental Health and Addictions Research Institute, said this may suggest participants developed a tolerance to fentanyl. Some participants reported actively seeking it out.

The study also found that non-prescribed opioid use was reported by 38 per cent of people in opioid substitution therapy, which involves treatment with drugs such as methadone, buprenorphine-naloxone (Suboxone) and hydromorphone. Fentanyl was detected in 52 per cent of these people in treatment.

The fact that people in substitution programs sometimes seek out other drugs shows that these programs may not be the "full answer" for this population, Dr. Honer said.

"What happens then is they're getting something stronger than what's prescribed," Dr. Honer said. "It could be a challenge to [substitution therapy] because the stuff that's prescribed won't work as well any more when they've been exposed to a higher-potency fentanyl drug. The good stuff – the prescribed stuff, the 'clean' stuff – may not be as effective."

Keith Ahamad, an addictions physician at St. Paul's Hospital and the B.C. Centre for Excellence in HIV/AIDS, viewed the findings differently.

"In this superhard to treat population, half the people aren't using [illicit] opioids, which is a major success," Dr. Ahamad said. He added that there is no evidence to suggest that higher opioid tolerance due to fentanyl may make substitution therapy less effective.

"It's true that all these medications have never been tested in the context of a drug market that is predominantly fentanyl, but it's still just an opioid – and the goal of these medications is to get rid of cravings, decrease [illicit] drug use and [maintain] tolerance, which decreases mortality for people who use drugs," he said. "If people stopped taking these medications, the mortality rate would skyrocket."

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Dr. Honer said his hope is that the study can serve a sentinel function to identify level of risk in individual communities.

Data released by the provincial Coroners Service on Wednesday showed that at least 1,422 people died of illicit drug overdoses in B.C. last year – a figure that is six times the annual average in the 2000s. Fentanyl was detected in 81 per cent.

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