Herbal-Drug Interactions
Herbal supplements are widely used but poorly regulated. St. John's wort, ginkgo, garlic, and many others cause clinically significant drug interactions that patients rarely report.
The Hidden Risk
Over 25% of adults use herbal supplements, yet fewer than half disclose this to their healthcare providers. Unlike prescription drugs, herbal products are not required to undergo rigorous interaction testing before marketing. Batch-to-batch variability in active compound concentrations adds further unpredictability.
St. John's Wort (Hypericum perforatum)
The most well-documented herbal interaction offender. St. John's wort potently induces CYP3A4, CYP2C9, CYP1A2, and P-glycoprotein through PXR activation — a profile comparable to rifampin.
Drugs with dangerous interactions:
- Cyclosporine/tacrolimus — multiple cases of transplant rejection
- HIV protease inhibitors — subtherapeutic levels and treatment failure
- Oral contraceptives — breakthrough bleeding and unintended pregnancy
- Warfarin — reduced INR and increased thrombosis risk
- SSRIs — serotonin syndrome risk (St. John's wort is also serotonergic)
- Digoxin — reduced levels via P-gp induction
Effects develop over 1-2 weeks and persist for a similar period after discontinuation.
Ginkgo biloba
Ginkgo inhibits platelet-activating factor (PAF) and has mild CYP-modulating properties:
- Anticoagulants/antiplatelets — increased bleeding risk; case reports of intracranial hemorrhage with warfarin
- NSAIDs — additive bleeding risk
- CYP2C19 substrates — ginkgo may inhibit CYP2C19, increasing levels of omeprazole and diazepam
- Seizure threshold — ginkgotoxin (present in seeds, occasionally in leaf extracts) may lower seizure threshold
Garlic Supplements
Concentrated garlic supplements (not culinary garlic) affect drug metabolism:
- CYP3A4 substrates — modest induction; reduced saquinavir levels by 50% in pharmacokinetic studies
- Anticoagulants — garlic inhibits platelet aggregation; additive bleeding risk
- Hypoglycemic agents — mild additive blood sugar lowering
Other Clinically Relevant Herbals
| Herbal | Key Interaction | Mechanism |
|---|---|---|
| Kava | Hepatotoxicity with hepatotoxic drugs | Additive liver injury; CYP2E1 inhibition |
| Ginseng | Reduced warfarin effect | Possible CYP induction; uncertain mechanism |
| Echinacea | CYP3A4 modulation (variable) | May inhibit or induce depending on preparation |
| Valerian | Additive CNS depression | GABAergic activity |
| Goldenseal | CYP2D6 and CYP3A4 inhibition | Berberine and hydrastine are potent inhibitors |
| Milk thistle | Mild CYP3A4/2C9 inhibition | Silybin is the active component; clinically modest |
Clinical Recommendations
- Always ask about herbal supplements during medication reconciliation
- St. John's wort should be treated as a strong inducer equivalent to rifampin in interaction assessments
- Discontinue herbals at least 2 weeks before elective surgery (bleeding risk with ginkgo, garlic, ginseng)
- Avoid herbals in transplant patients, those on anticoagulants, and HIV patients on antiretrovirals
- Check Natural Medicines database (naturalmedicines.therapeuticresearch.com) for evidence-graded interactions
Key Takeaways
- St. John's wort induces CYP3A4 and P-gp as potently as rifampin; transplant rejection and contraceptive failure are documented
- Ginkgo increases bleeding risk with anticoagulants and antiplatelets
- Fewer than half of herbal supplement users tell their doctors
- Goldenseal inhibits CYP2D6 and CYP3A4, potentially raising levels of many drugs
- Discontinue herbals 2 weeks before surgery