Herb-Drug Interactions: Common Examples and How to Screen for Them

An herb-drug interaction happens when an herbal supplement changes how a medicine is absorbed, broken down or cleared by the body, so the person gets too much or too little of the drug. St. John's wort is the best-documented example: it can weaken birth control pills, warfarin, some HIV drugs, transplant drugs and several cancer medicines [S01][S02]. Ginkgo with warfarin raises bleeding risk, and goldenseal lowered metformin levels by about 25 percent in a study of healthy adults [S01]. The practical fix is simple: record every supplement a patient takes, and check it against every medicine before anything changes.

Why herbs interact with drugs

Herbs contain active compounds, and those compounds use the same pathways in the body as medicines do. The FDA explains that some supplements can change the absorption, metabolism or excretion of a medication, which affects its potency [S03].

Two mechanisms come up most often:

• Liver and gut enzymes. Many drugs are broken down by cytochrome P450 enzymes such as CYP3A4. An herb that speeds these enzymes up lowers drug levels. An herb that blocks them raises drug levels.
• Drug transporters. Proteins such as P-glycoprotein move drugs into and out of cells. Changing their activity changes how much drug reaches the blood.

St. John's wort does both. NCCIH describes it as a potent inducer of cytochrome P450 enzymes and intestinal P-glycoprotein, which is why it has "an overall high risk of drug interaction" [S01].

Food works the same way. The FDA notes that grapefruit juice blocks intestinal CYP3A4, so more of some drugs, including certain statins, enter the blood. For a few other drugs, such as fexofenadine, it has the opposite effect and less drug is absorbed [S04].

A second kind of interaction has nothing to do with enzymes. It happens when an herb and a drug have the same effect, so the effects add up. The FDA gives the example of warfarin, ginkgo, aspirin and vitamin E, which can each thin the blood. Taken together, they may raise the risk of internal bleeding or stroke [S03].

Common herb-drug interactions

The table below lists interactions described by NCCIH and the FDA. It is a starting point for screening. It does not cover every interaction.

How strong is the evidence?

NCCIH is direct about this. Many herb-drug interactions listed in reference sources are "hypothetical," based on animal studies, cell experiments or indirect evidence, and well-designed clinical studies are limited and sometimes inconclusive [S01].

That does not mean the risk can be ignored. NCCIH says the issue needs attention for drugs with a narrow therapeutic index, where a small change in blood level matters. It names cancer chemotherapy drugs, warfarin and digoxin [S01]. For these drugs, even a possible interaction is worth taking seriously.

A useful way to sort the evidence:

1. Documented in people. St. John's wort with cyclosporine, indinavir, oral contraceptives, warfarin, digoxin and benzodiazepines; ginkgo with warfarin; goldenseal with metformin [S01].
2. Mixed or preliminary. Asian ginseng with warfarin; cranberry with warfarin; chamomile with birth control [S01].
3. Theoretical. Based on mechanism or lab data only.

Who is most at risk

Some patients deserve a closer look:

• People on narrow-margin drugs: warfarin and other blood thinners, digoxin, cancer drugs, transplant drugs and some seizure drugs [S01][S02].
• People taking several medicines. Each extra product adds possible combinations.
• People having surgery. The FDA notes that clinicians may ask patients to stop supplements two to three weeks before a procedure to avoid changes in heart rate, blood pressure or bleeding [S03].
• Children, and people who are pregnant or breastfeeding. Children metabolize substances at different rates, and supplements should be discussed with a clinician during pregnancy and breastfeeding [S03]. NCCIH says goldenseal should not be used during pregnancy or breastfeeding or given to infants [S05].
• People who assume "natural" means safe. The FDA says plainly that natural does not always mean safe [S03].

Product quality adds a second risk

An interaction check only works if the label is accurate. NCCIH notes that some goldenseal products have been found to contain herbs or ingredients not listed on the label, in addition to or instead of goldenseal [S05]. Supplements are not approved by the FDA before they are sold, and manufacturers are responsible for their safety and labeling [S03][S05].

When a patient brings a product in, record the brand, the full ingredient list and the dose, not just the herb's common name.

How to screen for herb-drug interactions in practice

A consistent process catches more problems than memory does.

1. Ask about everything, every visit

Ask about prescription drugs, over-the-counter drugs, vitamins, minerals, herbs, teas, powders and anything taken "only sometimes." The FDA suggests patients bring a list with doses and frequency, or bring the bottles themselves in a bag [S03].

2. Record it in a structured way

Store supplements in the same medication list as prescriptions, with product name, ingredients, dose, start date and who recommended it. Practice software with a structured medication and supplement list makes this easier to check later. Our guide to choosing practice software covers what to look for.

3. Check before you add or change anything

Before recommending a botanical, look it up against every current medicine. Before a patient starts a new prescription, recheck their existing supplements. Pay special attention to narrow-margin drugs.

4. Coordinate with the prescriber and pharmacist

When an interaction is likely, contact the prescribing clinician or pharmacist and agree a plan: avoid the combination, separate the timing, adjust and monitor, or choose a different option. Document the decision in the chart.

5. Follow up

Ask about new symptoms at the next visit. Lower drug effect (for example, breakthrough symptoms) and higher drug effect (more side effects) can both point to an interaction.

Building this into your practice

Interaction screening belongs in the intake workflow, not left to individual memory. If you are setting up a new clinic, add a supplement and medication review to your intake forms and visit templates from day one. See our practice startup plan for the wider setup, and check what your state allows naturopathic doctors to prescribe or recommend in our guide to credentials and licensing.