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Quick answer: herb-drug interactions happen when an herbal supplement changes how a medicine is absorbed, broken down or works in the body, making it weaker or stronger. Some of the best-documented examples, according to the National Center for Complementary and Integrative Health (NCCIH), are St. John’s wort (which can weaken birth control pills, some heart, transplant and HIV medicines and blood thinners), ginkgo and garlic supplements (possible bleeding risk with blood thinners), goldenseal and licorice root. Culinary amounts of herbs in food are generally a different matter from concentrated supplements. Before starting any herbal supplement, show the label to your pharmacist or doctor.
Herbal supplements sit on store shelves next to vitamins, and many people assume plant-based products are gentle. Plants contain active chemicals, though, and some of those chemicals interact with prescription and over-the-counter medicines in ways that matter.
This guide explains how interactions work, lists well-documented examples from government health sources, and gives a simple routine for checking before you try something new. It is educational information, not medical advice. For the basics of cooking with herbs, brewing tea and buying supplements wisely, see our beginner’s guide to herbs.
Sources: National Center for Complementary and Integrative Health (NCCIH) “Herbs at a Glance” fact sheets and “Using Dietary Supplements Wisely”; NIH Office of Dietary Supplements (ODS); MedlinePlus (National Library of Medicine) herbs and supplements and drug information; U.S. Food and Drug Administration (FDA) consumer updates on grapefruit juice and drug interactions and on dietary supplements; FDA Safety Reporting Portal. Checked October 2026.
What Is a Herb-Drug Interaction?
A herb-drug interaction is a change in how a medicine works caused by an herbal product taken at the same time. The result can be a medicine that works less well, a medicine that becomes too strong, or added side effects when two substances have similar effects.
Three main ways interactions happen
- Changing how a drug is broken down. The liver uses enzymes, such as those in the cytochrome P450 family, to process many medicines. Some herbs speed up these enzymes, lowering drug levels; others slow them down, raising drug levels.
- Changing how a drug is absorbed or moved. Some herbs affect transport proteins in the gut that control how much of a drug enters the bloodstream.
- Adding to a drug’s effect. An herb with blood-thinning, sedating or blood-sugar-lowering effects can stack with a medicine that does the same thing.
Why the stakes can be high
Some medicines have a narrow window between an effective dose and a harmful one. Blood thinners, transplant drugs, seizure medicines, some heart medicines and some cancer and HIV treatments fall into this group, so even modest changes in blood levels can matter.
Food Amounts vs Supplements
Cooking with herbs and spices is a normal part of a healthy diet. The interaction concerns described by NCCIH mostly involve concentrated supplements, extracts and large amounts, which deliver far more of a plant’s active compounds than a sprinkle of dried oregano or a few basil leaves.
That said, a few foods do matter for specific medicines, which is why the examples below include grapefruit and vitamin K. When in doubt, ask your pharmacist whether your usual diet needs any adjustment.
Well-Documented Herb-Drug Interactions
The examples below come from NCCIH fact sheets and other federal health sources. This is not a complete list, and new research continues to appear.
| Herbal product | Possible interaction | Medicines involved (examples) |
|---|---|---|
| St. John’s wort | Can make many medicines less effective; may cause serotonin syndrome with some antidepressants | Birth control pills, cyclosporine, digoxin, warfarin, some HIV and cancer drugs, antidepressants |
| Ginkgo | May increase bleeding risk | Blood thinners and anti-platelet drugs |
| Garlic supplements | May increase bleeding risk; can reduce levels of some HIV drugs | Blood thinners, saquinavir |
| Goldenseal | Can slow enzymes that process many drugs, raising their levels | Many medicines metabolized by CYP2D6 and CYP3A4; metformin |
| Licorice root | Large amounts can raise blood pressure and lower potassium | Blood pressure drugs, diuretics, corticosteroids, digoxin |
| Green tea extract | Has been linked to rare liver injury; green tea can lower levels of nadolol | Nadolol and other medicines |
| Asian ginseng | May lower blood sugar | Diabetes medicines |
| Valerian and other sleep herbs | May add to drowsiness | Sedatives, sleep aids, alcohol |
St. John’s wort
St. John’s wort is the best-known example. NCCIH explains that it speeds up the breakdown of many drugs, which can make them less effective. Reported problems include breakthrough bleeding and unplanned pregnancy with birth control pills and reduced effectiveness of medicines that prevent organ transplant rejection. Combining it with certain antidepressants may cause serotonin syndrome, a potentially dangerous condition.
Ginkgo and garlic
Ginkgo supplements may raise the risk of bleeding, and NCCIH advises caution for people taking blood thinners or planning surgery. Garlic supplements may also have blood-thinning effects. Cooking with garlic is different from taking concentrated garlic pills.
Goldenseal
NCCIH notes that goldenseal can slow down enzymes that process many medicines, potentially raising drug levels. It is often sold combined with echinacea, so check combination product labels carefully.
Licorice root
Licorice root contains glycyrrhizin, which in large amounts or with long-term use can raise blood pressure and lower potassium, according to NCCIH. That can interfere with blood pressure treatment and become risky with diuretics or heart medicines. Some licorice candy in the United States is flavored with anise instead of licorice root, so read the label.
Foods That Interact With Medicines
A few everyday foods and drinks have well-documented interactions that are worth knowing.
Grapefruit
The FDA warns that grapefruit juice can affect how the body handles a number of medicines, including some cholesterol-lowering statins, some blood pressure drugs, some organ transplant drugs and some antihistamines. With some drugs, it raises blood levels; with others, it lowers them. Seville oranges and pomelos can act in a similar way. Many prescription labels and pharmacy leaflets note when grapefruit should be avoided.
Vitamin K foods and warfarin
MedlinePlus explains that people taking warfarin should keep their vitamin K intake consistent, because vitamin K affects how the drug works. Leafy greens are high in vitamin K, and so is parsley. People on warfarin do not usually need to avoid these foods, but sudden large changes, such as starting to eat big bowls of parsley salad daily, can affect blood tests. Talk to the clinician who manages the medication before changing your diet.
Alcohol
Alcohol interacts with many medicines and adds to the drowsiness caused by sedating herbs such as valerian or kava. Kava has also been linked to serious liver injury, and the FDA issued a consumer advisory about that risk.
Who Is Most at Risk?
Anyone can experience an interaction, but some groups need extra caution.
- People taking several medicines. More medicines mean more chances for interactions.
- Older adults. Bodies process drugs differently with age, and older adults often take multiple prescriptions.
- People with liver or kidney disease. These organs clear both drugs and herbal compounds.
- People taking narrow-window drugs. These include blood thinners, transplant drugs, seizure medicines, digoxin and some cancer and HIV treatments.
- Pregnant or breastfeeding people. Safety data for most herbal supplements in pregnancy is limited.
- Children. Doses and safety for children are often unknown.
- Anyone scheduled for surgery. Some supplements may affect bleeding or anesthesia.
Before surgery
Tell your surgeon and anesthesia team about every supplement you take, including teas taken for health reasons. Many surgical teams ask patients to stop certain supplements in advance; follow their specific instructions.
Why Supplement Labels Are Not Enough
In the United States, the FDA regulates dietary supplements differently from medicines. Under the Dietary Supplement Health and Education Act of 1994, manufacturers do not need FDA approval before selling a supplement, and they are responsible for ensuring their products are safe and labeled accurately.
What that means for you
- Labels may not list interaction warnings.
- The amount of active compounds can vary between brands and even between batches.
- Some products have been found to contain ingredients not listed on the label, according to FDA warnings.
Quality seals
Seals from independent testing programs, such as USP Verified or NSF certification, indicate a product was tested for identity, purity or label accuracy. A seal does not mean the product is safe for you or free of interactions.
How to Check for Interactions Safely
A simple routine helps you catch problems before they start.
- Keep a complete list of every prescription, over-the-counter medicine, vitamin, herbal supplement and health tea you take, with doses.
- Ask your pharmacist before starting any new supplement. Pharmacists have interaction-checking tools and can usually answer quickly.
- Bring the actual bottle to appointments so your clinician can read the full ingredient list.
- Use one pharmacy where possible, so your records are in one place.
- Look up herbs in reliable sources such as the NCCIH “Herbs at a Glance” pages and MedlinePlus.
- Start one product at a time so you can tell what is causing any change.
- Never stop or change prescribed medicines to make room for a supplement without talking to the prescriber.
Signs that something may be wrong
Contact a clinician if you notice unusual bruising or bleeding, a racing heart, dizziness, extreme drowsiness, confusion, yellowing skin or eyes, dark urine, or symptoms returning that your medicine normally controls. Call emergency services for severe reactions.
Reporting problems
The FDA encourages consumers and clinicians to report serious side effects from dietary supplements through its Safety Reporting Portal or MedWatch. Reports help regulators spot unsafe products.
Questions to Ask Your Pharmacist
A short conversation with a pharmacist is the quickest way to check a supplement. These questions help you get a clear answer.
- Does this herb or any ingredient in this blend interact with my current medicines?
- Could it change how well my medicine works, or raise its side effects?
- Is there a safer time of day to take it, or should I avoid it entirely?
- Does this product contain more than one herb, and do any of them matter for me?
- What warning signs should make me stop and call you or my doctor?
- Are there food or drink changes I should make with my medicines, such as grapefruit or alcohol?
Write the answers down and keep them with your medication list so you have them for future appointments.
Herbs, Self-Care and Common Sense
Herbs can be part of a balanced self-care routine: a cup of peppermint tea after dinner, fresh herbs in your cooking, a fragrant pot of basil on the windowsill. Concentrated supplements are a different category that deserves the same care you give medicines.
For gentle, everyday habits that do not involve supplements, browse our complete self-care guide and our tips on how to improve your diet. To learn about traditional uses of individual plants, see our list of the most useful herbs in apothecary.
Frequently Asked Questions
Can herbal supplements interact with prescription drugs?
Yes. NCCIH documents interactions for several common herbal supplements, including St. John’s wort, ginkgo, goldenseal and licorice root. Some make medicines less effective, while others raise drug levels or add to side effects.
Which herbal supplement has the most drug interactions?
St. John’s wort is the best-known example. It can reduce the effectiveness of many medicines, including birth control pills, some transplant, heart and HIV drugs and warfarin.
Is it safe to drink herbal tea while taking medication?
Many mild culinary herbal teas are commonly used without problems, but some herbs used in teas, such as St. John’s wort, licorice root and goldenseal, can interact with medicines. Ask your pharmacist about any tea you drink regularly for health reasons.
Does cooking with herbs cause drug interactions?
Normal culinary amounts are generally a different matter from concentrated supplements. A few foods do matter for specific drugs, such as grapefruit with some medicines and vitamin K-rich foods with warfarin.
Should I stop herbal supplements before surgery?
Tell your surgical team about every supplement you take. Many teams ask patients to stop certain supplements beforehand, so follow their instructions.
Where can I check herb-drug interactions?
Your pharmacist is the fastest resource. Reliable free references include the NCCIH “Herbs at a Glance” fact sheets and MedlinePlus from the National Library of Medicine.
Sources
- National Center for Complementary and Integrative Health. Herbs at a Glance fact sheets (St. John’s Wort, Ginkgo, Garlic, Goldenseal, Licorice Root, Green Tea, Asian Ginseng, Valerian, Kava).
- National Center for Complementary and Integrative Health. Using Dietary Supplements Wisely.
- NIH Office of Dietary Supplements. Dietary supplement fact sheets.
- MedlinePlus, National Library of Medicine. Warfarin and vitamin K; herbs and supplements.
- U.S. Food and Drug Administration. Grapefruit Juice and Some Drugs Don’t Mix; dietary supplement regulation; kava consumer advisory.
- U.S. Food and Drug Administration. Safety Reporting Portal and MedWatch.
