The herb goldenseal, a commonly used natural supplement, may dull the effect of medication used to stabilize glucose levels in people with type-2 diabetes, according to Washington State University scientists.

For the study, researchers gave 16 healthy volunteers a low-dose cocktail of four drugs — which included the diabetes medication metformin — both before and after a five-day period of taking goldenseal. Each time volunteers received the cocktail, researchers took blood and urine samples to measure the presence of the drugs in their systems.

After volunteers returned for the second dose, following five days of ingesting goldenseal, researchers said participants had about 25% less metformin in their bodies than they did after the first dose. This likely means goldenseal has a depressive effect on the body’s ability to absorb metformin and other drugs that utilize the same drug transporters — proteins that allow the body to absorb or expel specific substances from cells.

Metformin and two other drugs included in the cocktail — the cholesterol-lowering drug rosuvastatin and furosemide, a diuretic — were intended to shed light on goldenseal’s effect on a host of drug transporters, but of the three, only metformin was affected.

“We’re pharmacokineticists, and so that means we study how the body handles drugs — we absorb, we distribute, we metabolize (and) we excrete drugs,” WSU Pharmaceutical Sciences professor Mary Paine said. “We look at mechanisms of all those different processes and how natural products can influence those processes.”

Paine, who is also principal investigator for the National Institutes of Health-funded Center of Excellence for Natural Product Drug Interaction Research, said the study is part of a larger body of work investigating the effect natural products may have on how the body handles prescribed medications.


She said there are well-established guidelines issued by the U.S. Food and Drug Administration for studying drug-drug interactions but there are no such guidelines for studying how natural products interact with pharmaceutical drugs. She said part of the purpose of the center is to better understand these interactions to help other researchers study them and ultimately to inform health care practitioners and patients about potential effects.

Other natural products being researched by the center include green tea, cannabis and kratom.

“Goldenseal isn’t prescribed by doctors or anything, it’s taken by patients on their own will,” said the study’s first author James Nguyen, a doctoral candidate in pharmaceutical sciences and recent doctor of pharmacy graduate. “If there’s information that’s going out to the media about that, then there’s this concern that they’re precipitating this type of interaction that may be decreasing the effects of their actual prescribed medication.”

Nguyen said goldenseal is a dietary supplement used to self-treat digestive ailments and is commonly used by people who have issues regulating their blood-glucose levels. While this most recent study establishes that goldenseal does have an effect on the absorption of metformin, he said it is uncertain whether this will have a noticeable effect on patients with type-2 diabetes who are being treated with the drug.

“We do have a follow-up study that we’re in the process of starting, which will ultimately confirm whether these findings in healthy volunteers are pertinent to diabetics,” Nguyen said. “That’s the ultimate goal is to see how (clinically) relevant these interactions we’re observing (are).”