Herbal remedies, such as St. John's wort, gingko biloba -- even garlic, may be putting patients on heart medications at serious risk, doctors are warning.

"It is potentially deadly and the most important thing is it's avoidable," Dr. Arshad Jahangir, one of the authors of the review and a consultant cardiologist at Mayo Clinic Arizona said.

In a scientific review in the Journal of the American College of Cardiology, doctors warn that while herbal remedies are thought of as safe and natural, they can cause serious interactions with heart drugs.

Some examples of supplements that can be dangerous to heart patients include:

• St. John's wort. It's typically used to treat depression, anxiety and sleep disorders, but it can reduce the effectiveness of heart medications, leading to recurrences of arrhythmia, high blood pressure or increase in blood cholesterol levels.

• Ginkgo biloba. This natural remedy is often taken to improve circulation or sharpen the mind, increases bleeding risk in those taking common blood thinners, such as warfarin or aspirin.

• Ginseng. While it's touted as a way to increase energy and mental alertness, it can also increase blood pressures, cause low blood sugar, and decrease the effects of warfarin.

• Green tea. While it's touted as an antioxidant and stimulant, green tea also contains vitamin K, so it too can make warfarin ineffective

• Garlic. Garlic supplements are often taken to lower cholesterol and blood pressure, which is important for heat patients. But at the same time, it can also increase the risk of bleeding among those taking warfarin.

The authors of the JACC review, who review over years of study on heart medication and herbal supplements, say the growing use of natural health products is especially concerning among elderly patients. That's because many of them have multiple health issues, take multiple medications, and are already at greater risk of bleeding.

"Many people have a false sense of security about these herbal products because they are seen as 'natural,'" Dr. Arshad Jahangir, one of the authors of the review and a consultant cardiologist at Mayo Clinic Arizona, said in a news release.

What's more, many patients don't realize that their health supplements could interact with their heart medications, so they don't alert their doctors to what they're taking, Jahangir and co-authors note.

"If patients aren't satisfied with their care today, many will turn to herbs because they believe these compounds can help them manage chronic conditions or improve health and prevent future disease," said Dr. Jahangir.

Dr. Nisha D'Mello of the University of Ottawa Heart Institute, who was not part of the JACC review, says up to 70 per cent of patients take supplements but many never tell their doctors and their doctors never ask.

"We, as physicians, always need to be asking these questions of our patients. Because there are a number of interactions which we haven't quite realized that occur with our prescription medications. And until we ask and find out, we will never know them," she told CTV News.

Jahangir believes that everyone – including patients, doctors, pharmacists and natural health care providers – have more to learn about the potential harm herbal remedies can have.

"These herbs have been used for centuries—well before today's cardiovascular medications—and while they may have beneficial effects, these need to be studied scientifically to better define their usefulness and, more importantly, identify their potential for harm when taken with medications that have proven benefit for patients with cardiovascular diseases," said Jahangir.

Naturopathic doctor Paul Saunders says patients considering herbal remedies should not try them without talking to an expert first.

"Herbals can be unsafe depending on the dose, the substance and your medical history," he says. "So it is important to consult a naturopathic doctor who has the training in herbal substances, who has the training in nutrition and who has the training in pharmacology to make sure that interactions don't happen."