Following a number of positive doping tests among prominent Canadian athletes, a University of Manitoba professor says it's possible for health supplements to be contaminated by banned substances.

Jay Johnson, an associate professor of kinesiology at the University of Manitoba who studies doping in sports, said some facilities that make supplements with steroids sometimes also make those that are labelled as free of steroids, and cross-contamination is possible.

He said the testing that takes place at the World Anti-Doping Agency has become much more sophisticated over the years and researchers are able to detect tiny amounts of trace elements that are likely in most people's bodies unknowingly.

"We're seeing more athletes in the last few years testing for substances where they're showing genuine surprise as to how they're getting into their bodies," Johnson told CBC News.

Many athletes are lying when they claim innocence after a positive doping test, but a handful have a legitimate claim, he said.

"They're taking a tainted supplement, essentially, that they didn't have awareness of, where the contamination wasn't listed (on the packaging)," Johnson said.

Canadians testing positive

On Monday, the Canadian Football League announced the suspension of Winnipeg Blue Bombers star Andrew Harris.

Harris won't play for two games after he tested positive for metandienone, also known as methandienone, an anabolic steroid.

The running back wouldn`t confirm the name of the product he used, but maintains his innocence.

"This is clearly a case of product contamination," he said, saying he was taking a male energy supplement with all-natural ingredients.

Canadian canoeist Laurence Vincent Lapointe has been suspended after testing positive for a banned substance. 3:12

Meanwhile, Canadian canoer and 11-time world sprint champion Laurence Vincent Lapointe was suspended two weeks ago over a positive test for the muscle-building substance Ligandrol.

She also said she believes a tainted supplement led to a positive drug test.

Taking responsibility

Professor Dean Kreillaars, another University of Manitoba professor in physical therapy, said nutritional supplements are required to be packaged with labels that clearly state their ingredients.

But that's not always the case with supplements produced in some other countries, he said.

"Whenever a person makes those attributes, they're typically trying to find an out," Kriellaars said. "You should be buyer beware and saying, 'Hey they're putting a real steroid inside of this and I shouldn't be taking it.' That's his responsibility. Even if he didn't know [what] was in it."

Johnson said athletes are in charge of what goes in their body, but cross-contamination isn't put on a label.

"How do you control for that level of contamination which has been shown to produce a handful of positive results for athletes in different sports?" he asked.

The issue of athletes being tripped up by contaminated supplements is not new.

Doping in other sports

In the Ultimate Fighting Championship, Jon Jones was allowed to compete earlier this year, after testing positive for trace amounts of a banned substance.

Jones, who was previously suspended for the drug back in 2017, was cleared by the U.S. Anti-Doping Agency because previous tests came back clean.

Jon Jones was allowed to compete earlier this year, after testing positive for trace amounts of a banned substance. (Erik Verduzco/Associated Press)

When Jones was suspended, he claimed it was due to taking off-brand Viagra which was contaminated.

A fellow fighter, Yoel Romero, won a major judgment after he ingested a tainted supplement.

At that time, Romero had the anti-doping agency test the supplement he had used. It concluded a banned human-growth hormone was present.

Romero sued the company, Gold Star Performance, and won the lawsuit worth more than $27 million after the company failed to respond.

Professor Jay Johnson said sometimes, growth hormones from meat can register positive results in athlete doping tests. (Jim Cole/Canadian Press)

Beyond supplements

In addition to supplements, Johnson said it's becoming more and more difficult to control what goes into what athletes eat.

He said some athletes and teams are bringing their own meat overseas for competitions to ensure growth hormones given to livestock like cattle don't end up in their bloodstream.

"It becomes very tricky to control for absolutely everything that goes into your body and your system," he said.

While instances of contamination are rare, Johnson said the public should reserve judgment until after all the facts are in.