Many complementary medicines beginning with ‘G’ have been associated with a risk of bleeding that pharmacists should know.
The G list
A few culprits come to mind. Ginkgo biloba contains ginkgolide B, which inhibits platelet-activating factor in vitro. In case reports, ginkgo has been associated with prolonged bleeding time.1,2
Garlic has been linked to decreased platelet aggregation and spontaneous bleeding; but a placebo-controlled study reported no change in INR with concomitant warfarin use.2
Ginger can inhibit platelet aggregation in vitro. A case report describes increased INR when ginger was combined with warfarin. However, controlled studies show no effect on warfarin pharmacokinetics or pharmacodynamics at recommended doses.1,2 Case reports have also shown increased INR in patients taking warfarin who started taking glucosamine.1,2 Panax ginseng contains antiplatelet components shown to inhibit platelet aggregation and thromboxane formation in vitro. Evidence on interactions with warfarin is conflicting.1,2 Goldenseal contains the alkaloid berberine, reported in in vitro and animal studies to inhibit platelet aggregation and increase bleeding risk.1
When risks stack up
While individual products might carry a modest bleeding risk at standard doses, the risk may increase when patients combine supplements, use higher doses or amounts, or use them alongside antiplatelet agents like aspirin or anticoagulants, such as warfarin.
Consider bleeding risk factors, including recent surgery, existing anticoagulation therapy, bleeding disorders or planned procedures.1
Practical recommendations
When dispensing anticoagulants or antiplatelet agents, use a non-judgemental approach to directly ask the patient if they are taking any complementary medicines. Asking about complementary medicine use is an important part of medication history taking.1 Document supplement use in the patient’s clinical record. Counsel patients to inform prescribers and surgeons about supplement use at least 2 weeks before elective procedures. Advise patients to watch for nosebleeds or unexplained bruising.1
What the evidence says
The clinical evidence for the bleeding risk of complementary medicines varies. The combination of in vitro data, case reports and individual variation in response supports caution.1,2 The Complementary medicines section of the Australian Pharmaceutical Formulary and Handbook contains useful information about bleeding risks and interactions with anticoagulants and antiplatelet agents for specific complementary medicines.
- Sansom LN, editor. Australian Pharmaceutical Formulary and Handbook. 2026. At: https://apf.psa.org.au
- Preston C, ed. Stockley’s drug interactions. London: Pharmaceutical Press; 2026
