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Herb–drug interactions (TCM)

Herbs used in traditional Chinese medicine, such as danshen, dong quai, ginkgo, ginseng and liquorice, have been reported to interact with conventional medicines, most often the anticoagulant warfarin, which has a narrow therapeutic range. Most of this evidence is case reports, laboratory and animal studies; the few randomised trials are small, of low quality and often find no significant effect, so for most individual herbs the size and frequency of any interaction is unknown. Two effects are better established: regular glycyrrhizin-containing liquorice raises blood pressure and lowers plasma potassium, and St John's wort (not a TCM herb, but the reference example) lowers blood levels of many medicines by inducing drug-metabolising enzymes.

Evidence retrieved up to

What the research shows

Most published reports of interactions between herbal medicines, including Chinese herbs, and conventional drugs are case reports, laboratory or animal studies; systematic reviews judge most case reports too poorly documented to evaluate and find few randomised trials, so for most herb-drug pairs the existence, size and frequency of an interaction in people is not established.

Moderate certaintyFurther research may change the size of the effect.
Take care: see the safety noteUnclear

Who this is about

adults using herbal medicines alongside conventional medicines

Practical note

This claim describes the evidence base rather than a health effect. Graded moderate: three systematic reviews spanning 2001-2015 agree that the literature is dominated by case reports and experimental studies; downgraded for indirectness, as the reviews cover different herb sets and only one is specific to Chinese herbs. A 2001 review scored only 13% of 108 suspected cases as well documented.

Safety note

Absence of trial evidence is not evidence of safety: reviews note that many herb-drug interactions probably go unrecognised. Pharmacists and prescribers can check specific combinations, especially for medicines with a narrow therapeutic range such as warfarin.

Sources

Among Chinese herbs, danshen, ginkgo, dong quai, American and Asian ginseng, safflower, peach kernel, liquorice, lycium (goji), ginger and notoginseng have the most reported evidence of interacting with warfarin; the reports describe raised INR and bleeding (for example with danshen and dong quai) or, for some herbs such as ginseng, a reduced warfarin effect, and come mainly from case reports and experimental studies.

Very low certaintyThis is uncertain; treat it as a lead, not a finding.
Take care: see the safety noteRisk

Who this is about

adults taking warfarin

Practical note

Graded very low: the evidence is mainly single case reports (for example one danshen case with INR above 8.4 and one dong quai case with INR up to 4.90), plus animal and in vitro work, and results conflict between herbs and study types. A 2021 systematic review of warfarin interactions also listed Chinese wolfberry, ginkgo, ginger and St John's wort among foods and herbs with increased bleeding risk.

Safety note

Warfarin has a narrow therapeutic range, so changes in its effect in either direction matter. Adults taking warfarin or other anticoagulants or antiplatelet drugs can tell their prescriber or pharmacist about any Chinese herbs, teas or formulas they take or start, since an INR check may be warranted.

Sources

Randomised trials of ginkgo, ginseng and other herbs taken with warfarin mostly found no significant change in warfarin's anticoagulant effect, although American ginseng altered it in one trial; the trials are few, small and of low methodological quality, so they neither confirm nor rule out the interactions described in case reports.

Low certaintyFurther research is likely to change this.
Take care: see the safety noteUnclear

Who this is about

adults taking warfarin, and healthy adult volunteers in pharmacology trials

Practical note

Graded low: a 2017 systematic review found nine randomised trials (mostly crossover designs, unclear risk of bias, CONSORT reporting 36.5-61.5%); downgraded for risk of bias and imprecision. Trials cited in a 2015 review used ginkgo 100 mg a day for four weeks and Panax ginseng extract 1.5 g a day for two weeks.

Safety note

A null result in a small short trial of one product does not show that every preparation of the herb is free of interaction; herbal products vary in composition.

Sources

Regular intake of liquorice products containing glycyrrhizin (glycyrrhizic acid) raises systolic and diastolic blood pressure and lowers plasma potassium, renin and aldosterone in trials, with the blood pressure rise larger at higher daily glycyrrhizin intakes; interactions between liquorice and corticosteroids have also been reported.

Moderate certaintyFurther research may change the size of the effect.
Take care: see the safety noteRisk

Who this is about

adults consuming glycyrrhizin-containing liquorice products regularly

Practical note

Graded moderate: two meta-analyses of trials agree in direction (18 studies, n = 337, mean rise 5.45 mm Hg systolic and 3.19 mm Hg diastolic with products providing at least 100 mg glycyrrhizic acid a day; and 26 trials, n = 985, diastolic rise 1.74 mm Hg); downgraded for small, heterogeneous trials. The evidence concerns liquorice itself; trials testing liquorice together with specific blood pressure or potassium-lowering medicines were not found.

Vegan note

Liquorice root and glycyrrhizin are plant-derived.

Safety note

Liquorice (gan cao) is common in Chinese herbal formulas. The blood pressure and potassium effects are most relevant to adults with high blood pressure, heart or kidney conditions, or taking medicines that affect blood pressure or potassium; NCCIH reports serious adverse effects such as irregular heartbeat with large or long-term intake. Deglycyrrhizinated liquorice lacks glycyrrhizin.

Sources

St John's wort, a herb used in several traditional systems though not central to TCM, is the best-documented example of herb-drug interaction by enzyme induction: by inducing cytochrome P450 enzymes (CYP3A4, CYP2C9, CYP1A2) and P-glycoprotein it lowers blood concentrations or effects of many medicines, including warfarin, ciclosporin, digoxin, HIV protease inhibitors and oral contraceptives.

Moderate certaintyFurther research may change the size of the effect.
Caution: see the safety noteRisk

Who this is about

adults taking prescription medicines

Practical note

Graded moderate: pharmacokinetic clinical trials consistently show lowered drug concentrations and regulators in the UK and Sweden judged the interactions clinically significant; downgraded because most trials measure blood levels rather than clinical outcomes, which rest largely on case reports. Included as the reference example of a mechanism (enzyme induction) that may also apply to other herbs.

Safety note

NCCIH describes St John's wort interactions as sometimes life-threatening, because it can weaken medicines such as transplant drugs, anticonvulsants, heart and HIV medicines and birth control pills. The degree of induction varies between preparations.

Sources