Popular weight-loss supplements containing green tea extract could be dangerous to your liver, an investigation by CBC's Marketplace reveals.

The supplements — which are licensed by Health Canada as "safe and effective" — are available over the counter at many popular retailers, including health stores and pharmacies.

But Marketplace discovered more than 60 documented cases worldwide of liver failure associated with green tea supplements reported in peer-reviewed journals.

At least two deaths have been partially linked to taking the pills.

"The risks are there and they are real," says gastroenterologist Dr. Herbert Bonkovsky, who has co-authored professional guidelines on green tea extracts and liver damage.

"People should not assume that because they are marketed as natural products that they are safe."

The market for weight management and well-being products in Canada is estimated to be worth about $304 million US a year, according to market research firm Euromonitor.

Browsing through the shelves of a pharmacy will reveal multiple weight-loss supplements containing green tea extract, with packaging that promises to "increase your fat burning ability," help "temporarily boost metabolic rate" and claim to be "clinically shown to promote fat oxidation."

Brewed beverage vs. extract

Joyce Boudreau-Hearn, from Mulgrave, N.S., died of complications from liver failure in 2010 after multiple transplants led to infection.

Her daughter, Jocelyn Stewart, says the 55-year-old had been healthy before she started taking a green tea extract sold as a weight-loss supplement.

"It says green tea extract — most of us link that to healthy," says Stewart. "Everybody buys a green tea; you can buy it anywhere.

"You think you will lose a few pounds. She lost her life."

Joyce Boudreau-Hearn died of complications from liver failure at age 55 after multiple transplants led to infection. Her daughter says the Nova Scotia woman was healthy before she started taking a green tea extract sold as a weight-loss supplement.

Green tea contains catechins, a type of antioxidant. And concentrated green tea extracts contain catechins at much higher levels than are found in the brewed version of the popular drink.

As the saying goes, there can be too much of a good thing. So while green tea is often regarded as good for you, Bonkovsky says green tea extract can be dangerous for some people at high doses.

Bonkovsky, along with other experts, isn't exactly certain why the extracts can affect some people's livers and not others. But generally he says high doses can pose a danger to the liver.

"If you take enough of it, it can kill you," Bonkovsky says.

Health Canada review

While adverse cases may be rare, the link between green tea extract and liver damage has been known for years. In 2003, France and Spain banned a green tea extract called Exolise after it was linked to liver damage.

If something is really difficult and we don't have terrific treatments for them, then the pharmacies and other places will be filled with these fake medications … - Dr. Sean Wharton, internal medicine specialist

Unlike prescription drugs, most natural health products don't require clinical trials to show they are safe and effective in order to be licensed by Health Canada.

Health Canada says the risk to your liver from green tea extract is rare, but the agency does require a warning on product packaging, which must state: "Consult a health-care practitioner prior to use if you have a liver disorder or develop symptoms of liver trouble (such as abdominal pain, dark urine or jaundice)."

Marketplace has learned that the agency is currently conducting a safety review of liver injury associated with all green tea extracts. A report is expected in March.

Questions about effectiveness

In addition to the potential for liver damage, there is a lack of solid evidence proving that the supplements are effective.

Marketplace looked at the research Health Canada used to determine that green tea extract products are effective treatments for weight management.

To evaluate the studies, Marketplace turned to McMaster University epidemiologist Jason Busse, who found the research couldn't determine whether green tea actually contributed to weight loss.

"All the studies have important limitations that preclude any confident conclusion that green tea preparations reduce weight," Busse says.

Health Canada says the risk to your liver from green tea extract is rare, but the agency does require a warning on product packaging. (CBC)

This comes as no surprise to Dr. Sean Wharton, the medical director of several publicly funded weight-loss clinics. He describes weight-loss supplements as "hope in a bottle."

"If something is really difficult and we don't have terrific treatments for them, then the pharmacies and other places will be filled with these fake medications to try to give people fake hope in an area where people are looking for an answer," he says.

Health Canada is also in the process of changing how natural products are licensed, and what claims they can make, after a 2015 Marketplace investigation showed problems with the current licensing system.

For that investigation, Marketplace submitted a made-up homeopathic children's cold and fever remedy to Health Canada, where it was approved and licensed as "safe and effective" without any scientific evidence.