Herb-Drug Interactions: How They Happen

Do not stop or change a prescribed medicine because of anything on this page. If you worry about an interaction, ask the prescriber or pharmacist first. MedlinePlus gives the same advice for a related risk: do not stop your medicine, and talk to your health care provider about your concerns first. [1]

What the words mean

• Enzymes are proteins that break medicines down. The family called cytochrome P450 (often written CYP) is the one these reviews name most. [5]
• Transporters are proteins that move medicines in and out of cells. P-glycoprotein is one named in the St. John's wort research. [6]
• Induction means an herb makes an enzyme or transporter work harder, so a medicine may be cleared faster and work less well. Inhibition means the opposite: a medicine may build up. [2]
• Pharmacokinetic describes what the body does to a medicine (absorption, breakdown, removal). Pharmacodynamic describes what the medicine and herb do together at their targets. A recent review of herbs and transplant medicines (Fitoterapia, 2026) uses both terms. [7]

What the research found

Documented, reported, theoretical

NCCIH uses different words for different levels of proof. For chamomile it says interactions with warfarin and some liver-processed drugs "have been reported," while interactions with sedatives are suspected for "theoretical reasons." For echinacea it describes "conflicting evidence" for liver-processed drugs. [12][13] A theoretical interaction is not proof of harm, and no report is not proof of safety. Our page on documented, reported and theoretical risks explains how to read these labels, and how interactions work covers the general picture.

Foods and drinks count too

A review of the tacrolimus literature lists grapefruit, pomelo, clementine, pomegranate, ginger and turmeric among CYP3A4 inhibitors. [14] MedlinePlus advises avoiding grapefruit juice or grapefruit with cyclosporine. [15] See grapefruit and medicine interactions.

Who should be most careful

NCCIH says dietary supplements may interact with your medications or pose risks if you have certain medical problems or are going to have surgery. [16] A review of tacrolimus, a transplant medicine, describes it as having a narrow therapeutic window and high variability between people, which is why changes in its blood level are a concern. [14] MedlinePlus lists herbal products that may interact with warfarin and with digoxin, and St. John's wort with cyclosporine. [17][18][15] NCCIH notes that many supplements have not been tested in pregnant women, nursing mothers or children. [16] See also older adults and interactions and pregnancy and breastfeeding.

Checking a combination

Proper evidence not found, in the sources we read, for any single database or "interaction checker" that is complete and always right. We did not evaluate any checker. A pharmacist can look up your exact medicines and products. Bring the labels. Useful starting points are how to check herb interactions with trusted databases and questions to ask your pharmacist. A printable medicine and supplement list worksheet helps.

Limits of what we know

The reviews we read summarize the literature; they cannot tell you how likely an interaction is for you. MedlinePlus reminds readers to tell a provider about any herbal medicines they take. [19] The most useful rule is simple: tell every clinician and your pharmacist about every herb, tea and supplement.