Chemists in Taiwan are warning people who rely on a common immunosuppressant medication to avoid candy flavored with licorice and licorice supplements because the herb appears to block the action of the drug.

Pei-Dawn Lee Chao, PhD, a chemist at China Medical University in Taichung, Taiwan and his team fed cyclosporine to laboratory rats with and without various doses of pure glycyrrhizin, an active compound in licorice root, and natural licorice extract. They say they were surprised to find that levels of cyclosporine dropped in the animals fed licorice or glycyrrhizin.

Cyclosporine is a drug primarily used by transplant patients to prevent organ rejection. But thousands also take it for rheumatoid arthritis, certain skin conditions and other diseases.

Licorice is a popular herb that’s been used in food and medicines for thousands of years. Its active ingredient, glycyrrhizin, is fifty times sweeter than sugar.

But not all licorice candy poses a threat. Many kinds of candy called “licorice” are actually flavored with the oil of a similar-tasting herb, anise.

Licorice is also a common herbal remedy for a variety of illnesses. Many people, for example, use licorice tablets to ease stomach irritation and heartburn.

Previous reports have indicated that licorice can trigger other potentially dangerous drug interactions. Some studies have shown licorice can interfere with the effectiveness of high blood pressure medications, aspirin, anti-inflammatory drugs, insulin and oral contraceptives.

This is the first time a drug interaction has been reported between licorice and cyclosporine.

To stay safe, experts advise avoiding foods or herbal products that list “licorice extract” or “licorice root extract” as ingredients.

The findings were presented this month at the National Meeting of the American Chemical Society in Salt Lake City.

Experts aren’t yet sure why licorice interferes with the immunosuppressant drug. There are no known scientific reports linking consumption of licorice to ill effects in transplant patients, but they add that doctors haven’t been looking for a link.

They also don’t know exactly how much licorice it takes to create a toxic effect and licorice-based products vary widely when it comes to their content of glycyrrhizin.

And other substances have been shown to reduce the absorption of cyclosporine. They include St. John’s Wort, onions, ginger, ginkgo and quercetin, which is found in onions, other plants and as a dietary supplement. Other studies have shown that certain substances, like grapefruit juice, actually boost cyclosporine levels.            

Robert Shaw, MD, is an instructor of Medicine at Johns Hopkins and a Rheumatologist at Carroll Arthritis clinic in Maryland. He says previous studies have also shown that serum concentrations of methotrexate, a common rheumatoid arthritis medication, are increased in rats with the use of licorice. But he had never heard of a problem with licorice and cyclosporine. “I would not have mentioned licorice in all honesty until someone put this (study) under my nose,” he said. “So it’s important for consumers of medications, even the lay person, to look into this as well.”

In light of this new study, Dr. Shaw says he will be telling his patients on cyclosporine to avoid licorice. “Suppose someone gave you a new kidney or a heart or lung transplant and you like licorice. Are you going to take the chance of impairing the effects of this incredible surgery by this candy? No,” he says with force. “You should avoid it.”

He says the same goes for rheumatoid arthritis patients on cyclosporine. “By the time we get to cyclosporine, we’ve gone through a lot of medications. And to take the chance of decreasing the usefulness of the medication would make your treatment very difficult."