Short answer: Several popular ones. The NCCIH herb-drug interaction digest rates St. John's wort and goldenseal as high risk because they change enzymes that handle many drugs. It reports goldenseal lowered metformin levels about 25%, high-dose green tea lowered the beta-blocker nadolol, and green tea extract lowered atorvastatin. Ginseng, cat's claw and chamomile have possible interactions with blood thinners and other drugs.
Many drugs are broken down by liver and gut enzymes in the cytochrome P450 family, such as CYP3A4 and CYP2D6, and moved by transporters such as P-glycoprotein. An herb that speeds these up can lower drug levels; one that slows them can raise drug levels. The NCCIH digest says goldenseal slows CYP2D6 and CYP3A4, which handle more than half of medicines in use, and that St. John's wort speeds up P450 enzymes and P-glycoprotein.
| Herb | Interactions described |
|---|---|
| St. John's wort | High overall risk. Documented with cyclosporine, indinavir, birth control pills, warfarin, digoxin and benzodiazepines; serotonin effects with some antidepressants |
| Goldenseal | High overall risk. Metformin levels fell about 25% in healthy adults, enough to potentially hinder blood sugar control |
| Green tea | High doses lowered nadolol (a beta-blocker); extract lowered atorvastatin (a statin) |
| Asian ginseng | Uncertain with blood pressure drugs, statins and some antidepressants; mixed results with warfarin |
| Cat's claw | May interact with blood thinners, antiplatelet and blood pressure drugs, CYP3A4 drugs and immunosuppressants |
| Chamomile | Preliminary studies suggest it might weaken birth control pills; interactions with warfarin reported |
The detail for St. John's wort is on its own page: St. John's wort and medicines.
Several herbs on this list touch warfarin. The MedlinePlus warfarin page lists coenzyme Q10, echinacea, garlic, ginkgo, ginseng, goldenseal and St. John's wort as non-prescription or herbal products that may interact with it, and says to tell your doctor and pharmacist about every vitamin and supplement. See vitamin K and warfarin and fish oil and blood thinners.
The digest also notes that cat's claw may slow blood clotting, which might increase bleeding during surgery.
Many entries in the digest are based on small or early studies, and it says there are theoretical reasons to suspect more interactions than have been shown. Products also differ: herbal supplements are not FDA-approved before sale, and NCCIH notes that the makers are responsible for their safety and labeling. A missing entry does not mean an herb is safe with your medicines.
NCCIH's advice is to tell your health care providers about any complementary product you use. A complete list with doses helps a pharmacist check it. Dose Track keeps prescriptions, injections and supplements in one list you can show at a visit.
Read more on Dose Track: Green tea extract · Panax ginseng · Cat's claw · Metformin side effects · Interaction checker