Chinese Herbs and Prescription Medicines: Interaction Concerns by Medicine Type

Urgent first

Unusual bleeding, black stools, vomiting blood, a severe headache or a fall with a head injury while taking a blood thinner and any herbal product needs emergency care. In the US call 911; for poisoning advice call Poison Help at 1-800-222-1222. Do not stop prescribed medicine on your own. Call the prescriber for advice.

What an interaction is

An interaction means one substance changes how another acts in the body. A review of cytochrome P450 enzymes (liver and gut enzymes that process many medicines) and Chinese herbs found that many herbs showed clinically relevant induction or inhibition of enzyme activity, particularly CYP3A4, with the potential to alter drug exposure, reduce effect or increase toxicity. The evidence it graded included laboratory assays, animal studies, human pharmacokinetic studies and case reports. [1] So most of what is known comes from laboratory and animal work and individual cases, not from large human trials. General background is on How Interactions Work.

Warfarin: the best-documented example

Warfarin has a narrow therapeutic window, which means small changes in its effect matter. A review of 64 articles (clinical reports, pharmacology and laboratory experiments) concluded that herbal medicines may strengthen or weaken warfarin through several mechanisms, that some herbs gave inconsistent results across study types, and that clinical use needs caution. [2] A Singapore review of 44 commonly used Chinese herbal products found 11 herbs with the strongest evidence of potential interaction: danshen, ginkgo, dong quai, American ginseng, safflower, peach kernel, licorice, Asian ginseng, lycium, ginger and notoginseng. [3] A separate review states that danshen is well known to lead to anticoagulation failure of warfarin. [4]

A hospital report of four cases (a single herb, a patent medicine, an injection and a compound decoction, each with warfarin) described fluctuations in INR, a blood test of clotting, accompanied by bleeding in some patients. The authors said the risk is easily overlooked by both Western and TCM doctors. [5] Four cases show that harm can occur; they cannot tell us how often.

Other medicine types

The NHS lists medicines that may not mix well with herbal medicines: statins, antidepressants, diabetes medicines, blood pressure medicines, warfarin and hormonal contraception. [6] We did not find a source that could responsibly rank herbs by risk for each of these medicine types, so "Proper evidence not found" applies to a complete list. The detailed pages cover diabetes and blood pressure medicines, mental health medicines and cancer treatment.

A second route to trouble is hidden drugs. NCCIH reports products contaminated with drugs such as warfarin and diclofenac, so a product may act like a medicine you did not know you were taking. [7] See Undeclared Drugs in Chinese Patent Medicines.

What this page cannot give you

There is no safe-combination list here, and there cannot be a reliable one: the sources above describe potential interactions, mixed results and individual cases. Products vary, doses vary, and people differ. A herb that is absent from a list is not proven safe.

Practical steps

• Tell every clinician and your pharmacist about every herbal product, tea or patent medicine, and show the labels. [7]
• Check with a pharmacist before starting a herbal medicine if you take other medicine, including over-the-counter products. [6]
• Tell your TCM practitioner every medicine you take, and ask them to coordinate with your doctor. See Telling Your Doctor About TCM Use.
• Do not stop or change a prescribed medicine yourself.

Evidence label: evidence of harm in individual cases; mixed and mostly preclinical for ranking specific herbs.