In the days since warning London marijuana users that their weed may contain fentanyl, the Middlesex-London Health Unit's medical officer of health has faced a "spirited" backlash.

Despite the skeptics, Dr. Chris Mackie said it would be irresponsible for him to not release information he says could protect Londoners from harm.

"As a public health practitioner, I'm trying to protect the health of 450,000 people. If there's a one per cent chance that there's fentanyl in someone's marijuana, that's a really serious risk for me."

In an Aug. 3 news release, the health unit warned that fentanyl was found in the urine of heroin and marijuana users who were screened at the Addiction Services Thames Valley Suboxone Clinic in July.  

In an interview on CBC's London Morning, Mackie called the possible presence of fentanyl in marijuana a "disturbing" finding, because it could expose new users to an opioid that's between 50 and 100 times more powerful than morphine and responsible for hundreds of fatal overdoses each year. 

CBC reached out to Health Canada for information about this story. A spokesperson replied with a written statement about testing done across the country.

"As of August 10, 2017, Health Canada only has needed to test marijuana for the presence of suspected fentanyl in a small number of samples. The analyses concluded that there was no fentanyl in the samples," read the statement in part.

Mackie also said marijuana users should carry naloxone, a drug that blocks opioid receptors and is used as an antidote for drug users who've overdosed on opioids. 

Fentanyl warning met with skepticism 

Not everyone appreciated Mackie's warning.

On CBC London's Facebook page, some commenters pounced.

"No drug dealer would waste expensive fentanyl on weed," wrote one.

"The users are lying about their drug use," wrote another. "Who the hell laces an expensive drug with another expensive drug?"

Today in an interview on London Morning, Nick Boyce of the Ontario HIV and Substance Use Training Program, suggested that Mackie's message, however "well-intentioned," may be misdirected.

He said if fentanyl is showing up in pot, it's likely in amounts too small to create the risk of an overdose or addiction. He also points out the original warning was based on self-reported — and therefore suspect — information from people in drug-treatment programs.

"We really need to focus the attention right now on getting naloxone into the hands of people who are most at risk of overdosing and those are people who are using opiate drugs already," he said.

Boyce said more overdoses would be prevented by making regular opioid users understand that they should not use illegal drugs alone and that they won't be charged if they report an overdose. He does agree with Mackie that expanding access to naloxone should be a priority. 

Not all the reaction was negative

Mackie stands by his decision to issue the warning, despite the "polarized" reaction it triggered. 

Also, he says not all the response was negative.

"I had people, random moms in shopping malls and coffee shops, come up to me and say 'Thank you for speaking out about this, drugs are a real risk and people seriously underestimate those risks.'"

He also said small amounts of fentanyl can cause a "serious reaction" in users and said people shouldn't put too much faith in drug dealers to deliver a pure product.

Alta Fentanyl Naloxone 20160512

London's chief medical officer of health says it's a good idea for recreational marijuana users to keep handy naloxone kits. (Mel Evans/Associated Press)

"I had someone say 'This would never happen except in some nefarious circumstances, if somebody did it on purpose,'" said Mackie. "But hey, that stuff happens. Drug dealers do bad things. People need to be aware of the risks and take some minor steps to protect themselves."

Among those steps is getting a naloxone kit (available free at most pharmacies, with a health card). And while that may seem ridiculous to the average weekend pot smoker, Mackie says it's a reasonable precaution.

"It is definitely possible to get a dangerous level of exposure accidentally in trace amounts," he said.

Contamination may be intentional, doctor says

Dr. Martyn Judson is an addiction specialist at London's Clinic 528, which has specialized in treating patients with opioid addiction since 2002. 

He says independent testing of his patients shows that other drugs, including fentanyl, are showing up in marijuana. So why is it there? He says it could be an intentional way for dealers to expand their clientele. 

"It's not enough to make the patient high or anything like that but it's just an infinitesimal amount that will make it that much more attractive to go back to that dealer," he said.

Mackie says he isn't scared by the backlash and is glad it's triggered some discussion about avoiding overdoses.

"I think if more people who are using marijuana recreationally had naloxone handy, I think that would be a really good thing for our community."