Herbs and medicines

The FDA puts the mechanism in one sentence: a supplement can change how a medication is absorbed, metabolized or excreted, so you get too much or too little of the drug you need. NCCIH describes the two directions. Some supplements increase a medication's effects, and its side effects. Others speed up the processes that break a drug into inactive substances, so the medicine quietly stops working.

Examples the sources name

These are combinations NCCIH and the FDA describe. The list is not complete; absence from it means nothing.

• St. John's wort reduces the effectiveness of many drugs; NCCIH names antidepressants, birth control pills, the transplant drug cyclosporine, heart medicines including digoxin, HIV and cancer medicines, warfarin and simvastatin. With certain antidepressants it can also increase serotonin-related side effects.
• Green tea, concentrated or at high doses, can reduce blood levels of the cholesterol drug atorvastatin and the beta-blocker nadolol.
• Goldenseal may change how the body processes many drugs; in an NCCIH-funded study it lowered levels of the diabetes medicine metformin by about a quarter.
• Ashwagandha has sedative effects and may strengthen benzodiazepines and other sedative or anti-anxiety medications.
• Warfarin, ginkgo, aspirin and vitamin E each thin the blood; the FDA warns that combining them can raise the risk of internal bleeding or stroke.
• Over-the-counter drugs count too: NCCIH notes that black cohosh and St. John's wort may interfere with the antihistamine fexofenadine.

NCCIH is candid about this research: well-designed clinical studies of herb-drug interactions are limited and sometimes inconclusive, and many reported interactions are inferred from animal or cell studies rather than seen in people. That cuts both ways: a missing warning is not a clean bill, because nobody may have looked.

No room for error

NCCIH singles out drugs with a narrow therapeutic range, where the dose must stay within tight limits to avoid serious reactions: warfarin, digoxin, cyclosporine, and the seizure medicines carbamazepine and phenytoin. With one of these, an herb is not a casual addition. NCCIH also flags growing concern that supplements may interfere with cancer and HIV care.

Tell everyone, every time

The most useful habit is a boring one. NCCIH says to tell all of your health care providers about every supplement and medicine you take, and to bring a written list: each product, how often, and the dose. The FDA's alternative is to put every bottle in a bag and bring the bag. Update the list at every visit, and include the pharmacist, especially before pairing a supplement with an over-the-counter drug.

Surgery, pregnancy, children

• Before surgery: supplements can affect your response to anesthetics and other drugs given around an operation, raise bleeding risk, or cause serious changes in heart rate or blood pressure. The FDA says you may be asked to stop them two or three weeks ahead; tell the surgical team what you take even in an emergency.
• Pregnancy and breastfeeding: many supplements have never been tested in pregnant women or nursing mothers; the FDA says to discuss any supplement with your health care professional first.
• Children: the FDA points out that children's metabolisms are unique and change with age, so combining supplements with medicines in a child carries a real possibility of harm.