Mixing some herbal remedies with heart medications may lead to bleeding or a higher risk of heart problems, a U.S. review has found.

Certain herbal supplements and foods — including St. John's wort, gingko biloba, garlic and grapefruit juice — can pose serious risks to people taking medications for heart disease, researchers say in the Feb. 9 issue of the Journal of the American College of Cardiology.

The mixing either reduces the effectiveness of conventional medications or increases their potency, the researchers said.

"We can see the effect of some of these herb-drug interactions — some of which can be life-threatening — on tests for blood clotting, liver enzymes and, with some medications, on electrocardiogram," study author Dr. Arshad Jahangir, a professor of medicine and a cardiologist at Mayo Clinic Arizona, said in a news release.

For example:

• St. John's wort, a supplement used to treat depression, anxiety and sleep disorders among other problems, reduces the effectiveness of some medications. This can contribute to recurrences of arrhythmia, high blood pressure or an increase in blood cholesterol levels and the risk of future heart problems.
• Ginkgo biloba, which is taken to improve circulation or sharpen the mind, increases the bleeding risk among people taking warfarin or acetylsalicylic acid.
• Garlic, which may help boost the immune system and lower cholesterol and blood-pressure levels, can increase the risk of bleeding among people on warfarin.

About half the patients in the research covered by the review did not tell their doctors they were taking supplements. The omission is a concern, given that many patients may be on multiple conventional medications, the authors said.

Nor do health-care providers routinely ask about use of supplements. Jahangir and the other authors called for more public education about the risks of herbal supplements, and more research into the effects of natural compounds on heart-disease management.

Herbs in the United States are considered food products and aren't subject to the same scrutiny and regulation as conventional medications, the authors noted.

Douglas MacKay of the Council for Responsible Nutrition, a trade group representing the dietary supplement industry, called the article biased. In a written statement, MacKay said that many herbal supplements offer healthful benefits, and the potential risk of drug interactions would be eliminated if patients spoke openly with doctors about supplements.

"If consumers are buying from reputable companies, they can feel confident in the quality and safety of these products," he said.

Some prescription medications may interact with grapefruit but most do not, the Florida Department of Citrus said in response to Jahangir's review.

The blood pressure medications Norvasc (amlodipine) and Cardizem (diltiazem) are generally considered to be safe to consume with grapefruit, as are the cholesterol-lowering statin drugs Pravachol (pravastatin), Lescol (fluvastatin) and Crestor (rosuvastatin), the citrus board said.