The 20 Most Common Herb–Drug Interactions: What Every Patient and Practitioner Should Know

Yoon Hang Kim, MD, MPH

Board Certified in Preventive Medicine | Integrative & Functional Medicine Physician

www.directintegrativecare.com

Introduction

Herbal supplements and botanical medicines are now used by an estimated one-third of American adults, yet the majority of patients never mention these products to their physicians. This communication gap creates a hidden layer of pharmacological complexity, because many widely available herbs and supplements can interact with prescription medications in clinically meaningful ways—sometimes with dangerous consequences.

The interactions described in this article span several major pharmacological mechanisms: induction or inhibition of cytochrome P450 (CYP) enzymes and P-glycoprotein (P-gp) transporters, competitive antagonism of vitamin K, additive effects on hemostasis, and pharmacodynamic synergism at central nervous system (CNS) receptors. Some of these interactions have been documented through rigorous clinical trials; others rest primarily on case reports and pharmacokinetic modeling. In all cases, the clinical principle is the same: any substance that enters the body has the potential to alter the behavior of other substances already there.

Below are twenty of the most commonly encountered herb–drug interactions, organized by therapeutic category. Each entry summarizes the mechanism, the clinical significance, and the supporting evidence.

Anticoagulants and Antiplatelets

1. Ginkgo biloba + Warfarin. Ginkgo biloba is among the most purchased dietary supplements in the United States, and its interaction with warfarin has been a subject of considerable investigation. A large Veterans Administration database study involving over 800,000 warfarin-treated patients found that concurrent ginkgo use was associated with a significantly increased risk of bleeding events (hazard ratio 1.38, 95% CI 1.20–1.58, p < 0.001). A 2025 retrospective analysis from Hai Phong International Hospital similarly identified significant correlations between ginkgo–anticoagulant co-administration and both clinical bleeding and abnormal coagulation parameters. The mechanism is multifaceted: ginkgolide B acts as a platelet-activating factor (PAF) antagonist, while the bilobalide component induces hepatic CYP enzymes including those metabolizing warfarin. Interestingly, some controlled studies using the standardized extract EGb 761 have not confirmed significant hemostatic impairment, suggesting that product variability may be an important factor. Clinicians should counsel all patients on warfarin to disclose ginkgo use and to monitor INR closely if concurrent use is continued.

2. Garlic (Allium sativum) + Warfarin and Antiplatelet Agents. Garlic supplements are widely used for cardiovascular health, and the active sulfur-containing derivative ajoene has been shown to irreversibly inhibit platelet aggregation. Case reports have documented elevated INR values in patients who combined garlic supplements with warfarin, and a comprehensive 2022 review in the Annals of Medicine and Surgery concluded that garlic and hawthorn supplementation should be discontinued prior to surgery because of their strong association with surgical bleeding. However, a double-blind, placebo-controlled pilot study of 48 patients found no evidence of increased hemorrhage when aged garlic extract (Kyolic) was used alongside warfarin for 12 weeks, suggesting that the form of garlic preparation matters. Clinicians should remain cautious, particularly with high-dose garlic supplements or raw garlic consumption in patients taking warfarin, aspirin, or clopidogrel.

3. Chamomile + Warfarin. Chamomile tea and supplements contain coumarin derivatives that may theoretically potentiate warfarin’s anticoagulant effect. Case reports have described elevated INR values and bleeding episodes in patients combining chamomile with warfarin. A 2021 systematic review classified chamomile as having a major-severity interaction with warfarin based on CYP2C9 inhibition and its intrinsic coumarin content. While the evidence base remains limited to case reports and pharmacological plausibility rather than controlled trials, it is prudent to advise patients on warfarin about the potential risk, particularly with concentrated chamomile supplements or frequent, high-volume chamomile tea consumption.

4. Green Tea (Camellia sinensis) + Warfarin. This interaction operates through a different mechanism than most herb–drug interactions: green tea leaves contain substantial amounts of vitamin K, which directly antagonizes warfarin’s anticoagulant effect. The index case involved a 44-year-old man on warfarin whose INR dropped from 3.79 to 1.37 after he began drinking approximately half a gallon to one gallon of green tea daily. Upon discontinuation of the green tea, his INR rebounded to 2.55. A pharmacodynamic study in volunteers confirmed a statistically significant decrease in both INR and prothrombin time when green tea was co-administered with warfarin. Importantly, brewed green tea contains far less vitamin K than dried tea leaves (approximately 0.03 µg per 100 ml of brewed tea versus over 1,400 µg per 100 g of dried leaves), so moderate consumption is unlikely to cause problems. Patients should be counseled to maintain consistent intake rather than making dramatic changes in green tea consumption.

5. American Ginseng (Panax quinquefolius) + Warfarin. American ginseng has been shown in clinical studies to modestly reduce the anticoagulant effect of warfarin. One study demonstrated that American ginseng at 1 g twice daily for three weeks modestly reduced the area under the curve (AUC) of warfarin in healthy volunteers with a slight decrease in INR. The mechanism may involve induction of CYP enzymes, though the ginsenosides present in ginseng have also been shown in some in vitro studies to inhibit CYP2C9 and CYP3A4, creating a complex pharmacological picture. The clinical evidence is mixed: some studies show no effect on warfarin pharmacokinetics, while others demonstrate modest INR reductions. Patients on warfarin who wish to use ginseng should have their INR monitored more frequently.

Immunosuppressants and Antiretrovirals

6. St. John’s Wort (Hypericum perforatum) + Cyclosporine or Tacrolimus. This is arguably the most dangerous herb–drug interaction in clinical medicine. St. John’s wort is a potent activator of the pregnane X receptor (PXR) and thus a powerful inducer of CYP3A4 and P-glycoprotein, both of which are critical to the metabolism and absorption of calcineurin inhibitors. Multiple case reports and case series have documented transplant rejection caused by subtherapeutic cyclosporine levels in patients who self-medicated with St. John’s wort. In a landmark set of cases, two heart transplant patients developed acute rejection confirmed by endomyocardial biopsy after starting 900 mg daily of a commercial St. John’s wort extract; cyclosporine concentrations returned to therapeutic range after the herb was discontinued. One renal transplant patient developed chronic rejection and returned to dialysis after a similar interaction. A comprehensive 2020 review in the British Journal of Pharmacology confirmed that the degree of CYP3A4 induction correlates with the hyperforin content of the preparation. Concurrent use is contraindicated.

7. St. John’s Wort + HIV Protease Inhibitors (e.g., Indinavir, Saquinavir). St. John’s wort’s induction of CYP3A4 dramatically reduces the plasma concentrations of HIV protease inhibitors. Clinical data have shown reductions in indinavir exposure of up to 57%, which is sufficient to cause virologic failure and the emergence of drug-resistant HIV. The FDA issued a public health advisory on this interaction, and all major HIV treatment guidelines explicitly contraindicate concurrent use. This interaction underscores why patients on antiretroviral therapy must be specifically asked about herbal supplement use at every visit.

8. St. John’s Wort + Combined Oral Contraceptives. CYP3A4 induction by St. John’s wort decreases serum concentrations of both the estrogen and progestin components of combined oral contraceptives, leading to breakthrough bleeding and risk of contraceptive failure. Case reports include unintended pregnancies in women taking St. John’s wort concurrently with oral contraceptives. Two well-designed pharmacokinetic studies have confirmed significantly reduced estrogen and progestin levels during concurrent use. Patients using hormonal contraception should be advised to use backup barrier methods if they choose to take St. John’s wort, or ideally to avoid the combination altogether.

9. St. John’s Wort + Warfarin. In addition to its effects on cyclosporine and protease inhibitors, St. John’s wort’s induction of CYP enzymes (particularly CYP2C9 and CYP3A4) and P-glycoprotein can reduce warfarin levels and decrease its anticoagulant effect. Clinical reports have documented decreased INR values in patients who added St. John’s wort to a stable warfarin regimen. The 2020 British Journal of Pharmacology review listed warfarin among the drugs whose pharmacokinetics are significantly altered by St. John’s wort. Patients on warfarin should avoid this herb, and if concurrent use has occurred, INR should be monitored closely during and after discontinuation to avoid both subtherapeutic anticoagulation and rebound overanticoagulation.

10. St. John’s Wort + Benzodiazepines (e.g., Alprazolam, Midazolam). CYP3A4 induction by St. John’s wort can reduce plasma concentrations of benzodiazepines metabolized by this pathway. In a study of healthy subjects, 14 days of St. John’s wort pretreatment reduced alprazolam plasma concentrations by more than 50%, a reduction large enough to substantially diminish anxiolytic efficacy. Similar effects are expected with midazolam and other CYP3A4-dependent benzodiazepines. Pharmacists and prescribers should be alert for evidence of reduced benzodiazepine effect when patients are also taking St. John’s wort.

Cardiovascular Drugs

11. Hawthorn (Crataegus spp.) + Beta-Blockers. Hawthorn extract has documented inotropic and vasodilatory properties, and its concurrent use with beta-blockers such as atenolol or propranolol may produce additive hemodynamic effects. The clinical concern is potentiation of hypotension and bradycardia. A 2022 review in the Annals of Medicine and Surgery identified hawthorn as having a strong association with surgical bleeding independent of anticoagulants, and recommended discontinuation prior to surgery. While large-scale controlled trials specifically examining the hawthorn–beta-blocker combination are lacking, the pharmacological plausibility of additive cardiodepressant effects warrants cautious co-administration and close blood pressure and heart rate monitoring.

12. Hawthorn + Digoxin. Hawthorn’s positive inotropic effects and its potential to alter cardiac glycoside pharmacodynamics raise concern for additive toxicity when combined with digoxin. Theoretical mechanisms include effects on cardiac ion channels similar to those of digoxin itself. Given digoxin’s narrow therapeutic index, even modest potentiation of its effects could increase the risk of arrhythmia or toxicity. Clinical guidelines recommend close monitoring of digoxin levels and cardiac rhythm in patients who are using hawthorn concurrently.

13. Hawthorn + Calcium Channel Blockers. The vasodilatory and chronotropic-reducing properties of hawthorn may produce additive effects when combined with calcium channel blockers such as diltiazem, nifedipine, or verapamil. The potential for excessive blood pressure reduction and symptomatic hypotension is of primary concern. As with the hawthorn–beta-blocker interaction, the evidence is largely based on pharmacological reasoning and clinical extrapolation rather than large clinical trials, but the principle of additive hemodynamic depression is well established in cardiovascular pharmacology.

Metabolic and Endocrine Drugs

14. Goldenseal (Hydrastis canadensis) + Metformin. Goldenseal contains the alkaloids berberine and hydrastine, both of which have been shown to affect drug transporters and CYP enzymes. A 2021 clinical study in healthy adults demonstrated that goldenseal decreased metformin systemic exposure by approximately 23%, mediated through inhibition of intestinal uptake transporters (OCTs and MATEs). A subsequent 2025 three-arm crossover study in adults with type 2 diabetes found that the magnitude of this interaction was metformin dose-dependent: approximately 20% reduction at low metformin doses (500–750 mg), 14% at moderate doses, and negligible at high doses. Interestingly, HbA1c decreased from 6.8% to 6.5% regardless of the pharmacokinetic interaction, suggesting potential independent glucose-lowering effects of goldenseal. Healthcare providers should counsel patients about the unpredictable effects of combining goldenseal with metformin therapy.

15. Goldenseal + CYP2D6 and CYP3A4 Substrates. Goldenseal is one of the few herbal products for which robust clinical pharmacokinetic data exist regarding enzyme inhibition. Studies led by Gurley and colleagues have confirmed that a 900 mg goldenseal extract administered three times daily for 28 days inhibited CYP2D6 and CYP3A4 activity by approximately 40%. A 2017 review classified goldenseal’s overall drug interaction risk as high because CYP2D6 and CYP3A4 together are responsible for the metabolism of more than half of all currently used pharmaceuticals. Clinically, this means that goldenseal has the potential to raise plasma concentrations of numerous drug classes, including beta-blockers, antiarrhythmics, antidepressants (SSRIs, tricyclics), antipsychotics, benzodiazepines, statins, calcium channel blockers, and opioid analgesics. Any patient taking medications metabolized by these pathways should be advised about the interaction risk.

16. Asian and American Ginseng + Antidiabetic Agents. Both Panax ginseng (Asian) and Panax quinquefolius (American) have been shown to have hypoglycemic effects, primarily through modulation of insulin secretion and glucose uptake. When combined with antidiabetic medications such as insulin, sulfonylureas, or metformin, additive blood glucose-lowering effects can increase the risk of clinically significant hypoglycemia. Clinical study results are mixed—some show modest glucose reduction while others show minimal effect—but the pharmacological plausibility of additive hypoglycemia warrants counseling patients with diabetes who use ginseng products to monitor blood glucose more frequently and to inform their prescribers.

CNS and Psychotropic Drugs

17. St. John’s Wort + SSRIs/SNRIs. Combining St. John’s wort with serotonergic antidepressants creates a dual risk: pharmacodynamic synergism of serotonergic activity (risking serotonin syndrome) and pharmacokinetic alteration through CYP enzyme induction. Serotonin syndrome is a potentially life-threatening condition characterized by agitation, hyperthermia, diaphoresis, tachycardia, hyperreflexia, and in severe cases, seizures and rhabdomyolysis. Multiple case reports have documented this combination precipitating serotonin syndrome. Given the severity of the potential outcome and the availability of safer alternatives for mild depression, this combination should be avoided.

18. Valerian (Valeriana officinalis) + Benzodiazepines and Other Sedatives. Valerian root has a long history of use as a sedative and anxiolytic, and its active constituents—particularly valerenic acid—act as positive allosteric modulators of GABA-A receptors, the same receptor complex targeted by benzodiazepines. A clinical case report documented a patient on lorazepam who developed handshaking, dizziness, and muscular fatigue within 32 hours of adding valerian and passionflower, attributed to an additive or synergistic effect on GABAergic neurotransmission. The NIH Office of Dietary Supplements advises that individuals taking benzodiazepines, barbiturates, or other CNS depressants should be aware of the theoretical possibility of additive sedation with valerian. Although a 2014 evidence review concluded that warnings regarding clinically relevant valerian interactions lacked strong recent evidence, the pharmacological rationale for additive CNS depression remains sound, and caution is appropriate.

19. Kava (Piper methysticum) + Benzodiazepines or Barbiturates. Kava’s active constituents, the kavalactones, modulate GABA-A receptors and inhibit noradrenaline reuptake, producing anxiolytic and sedative effects that can be additive with benzodiazepines and other CNS depressants. A frequently cited case report described a 54-year-old man hospitalized in a semicomatose state attributed to the combined use of kava and alprazolam for three days. The NIH National Center for Complementary and Integrative Health advises that kava should not be used together with other substances that have sedative effects. Beyond the sedation risk, kava carries an independent concern for hepatotoxicity, which led to regulatory action in multiple European countries in the early 2000s. The FDA issued a consumer advisory in 2002 regarding the potential risk of liver damage. Patients using kava should be advised against concurrent use with benzodiazepines, alcohol, or any hepatotoxic medication.

20. Green Tea (High-Dose) + Nadolol (Beta-Blocker). While moderate green tea consumption interacts with warfarin primarily through vitamin K content (discussed above), high-dose green tea has also been shown to reduce blood levels of the beta-blocker nadolol. The proposed mechanism involves inhibition of organic anion transporting polypeptide (OATP) uptake transporters in the intestine, reducing nadolol absorption. The NIH’s clinical digest on herb–drug interactions specifically highlights this interaction, noting that green tea at high doses has been shown to reduce blood levels and therefore the effectiveness of nadolol. This interaction is clinically relevant because reduced nadolol levels could lead to inadequate blood pressure or heart rate control. Patients taking nadolol or similar beta-blockers should be counseled about maintaining consistent and moderate green tea consumption.

Clinical Takeaways for Practitioners

Several practical principles emerge from this review. First, every clinical encounter should include a specific question about herbal supplement and botanical medicine use—studies consistently show that the majority of patients do not volunteer this information unless directly asked. Second, the pharmacological mechanisms underlying these interactions are well characterized and predictable: CYP enzyme induction (St. John’s wort), CYP enzyme inhibition (goldenseal, grapefruit juice), P-glycoprotein modulation, vitamin K antagonism, additive hemostatic effects, and additive CNS depression account for the vast majority of clinically significant herb–drug interactions.

Third, product variability is a critical and often underappreciated factor. The hyperforin content of St. John’s wort preparations, the form of garlic supplement (aged extract versus raw), and the concentration of kavalactones in kava products all influence the magnitude of potential interactions. Standardized, well-characterized products are not equivalent to unstandardized products, and clinical data from one preparation may not apply to another.

Finally, several free and subscription-based interaction-checking tools are available to assist clinicians: the Memorial Sloan Kettering About Herbs database, the NIH’s NCCIH clinical digest on herb–drug interactions, the NatMed Pro (Natural Medicines) database, and standard drug interaction checkers such as those on Medscape and Drugs.com all provide varying levels of herb–drug interaction coverage.

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© 2026 Yoon Hang Kim, MD, MPH – Direct Integrative Care

www.directintegrativecare.com