Herbal medicines are often described as natural, traditional, or plant-based. This can make them seem harmless. However, a product being natural does not automatically mean it is safe. Herbal products contain active chemical substances that can affect the body. Some of these substances can change how prescription medicines work (National Center for Complementary and Integrative Health [NCCIH], 2024).
An interaction may make a prescription medicine less effective, increase its effects, cause unexpected side effects, or raise the risk of serious harm. Interactions can occur with herbal capsules, tablets, powders, teas, tinctures, oils and concentrated extracts. They may also involve over-the-counter medicines, vitamins and other dietary supplements.
Not every suspected herbal interaction has been proved through strong clinical research. For many herbal products, the evidence is limited, mixed or based on laboratory studies and individual case reports. Nevertheless, several clinically important interactions have been documented, particularly with St. John’s wort and medicines such as warfarin and cyclosporine (NCCIH, 2024).
What Is an Herbal Medicine?
An herbal medicine is a product made from a plant or part of a plant and used in an attempt to prevent, relieve or treat a health problem. It may contain leaves, flowers, seeds, roots, bark, berries or plant extracts.
Examples include:
- St. John’s wort
- Ginkgo biloba
- Garlic supplements
- Ginseng
- Goldenseal
- Valerian
- Echinacea
- Turmeric or curcumin products
- Concentrated green tea extracts
- Traditional herbal mixtures
The word herbal does not describe one standard type of product. Different products made from the same plant may contain different ingredients or concentrations. A tea, powder, capsule and concentrated extract made from the same herb may not have the same strength or effect.
Using an ordinary amount of a herb in food is also not necessarily the same as taking a concentrated supplement. For example, garlic used in cooking usually produces much lower exposure than a concentrated garlic extract taken every day.
What Is a Medicine Interaction?
A medicine interaction happens when one substance changes the way another substance works in the body. The interaction may involve two prescription medicines, a prescription medicine and an over-the-counter product, or a conventional medicine and an herbal product.
An interaction can cause the prescription medicine to:
- Become less effective
- Become more powerful than expected
- Remain in the body for too long
- Leave the body too quickly
- Produce additional side effects
- Affect an organ differently
- Combine with the herb to create a new risk
The U.S. Food and Drug Administration (FDA, n.d.) explains that supplements can change the absorption, metabolism or excretion of medicines. This may leave the body with too much or too little of the prescribed medicine.
How Do Herbal Medicines Interact With Prescription Medicines?
Herbal products can interact with medicines in several ways.
1. They can change how a medicine is absorbed
A medicine must normally be absorbed from the digestive system before it can circulate through the body. An herbal product may reduce, delay or increase this absorption.
If too little medicine is absorbed, the treatment may not work properly. If more medicine than expected is absorbed, the risk of side effects may rise.
An interaction involving absorption does not always occur immediately. Its importance depends on the herb, medicine, dose and timing.
2. They can change how the liver processes medicines
The liver uses enzymes to process many medicines. One important group is the cytochrome P450 enzyme system. Herbal substances can increase or reduce the activity of these enzymes.
An herb that speeds up an enzyme may cause the body to break down a medicine too quickly. The amount of medicine in the blood can fall, making the treatment less effective.
An herb that slows an enzyme may cause the medicine to accumulate. This can increase side effects or toxicity.
St. John’s wort is a major example. It increases the activity of several drug-processing pathways, including cytochrome P450 enzymes and the transporter P-glycoprotein. As a result, it can reduce the blood levels and effectiveness of many medicines (NCCIH, 2024).
3. They can affect drug transporters
Drug transporters are proteins that move medicines into or out of cells. They can affect how much medicine is absorbed, reaches a particular organ or is removed from the body.
Some herbs influence transporters such as P-glycoprotein. A change in transporter activity may alter the amount of medicine available to produce its intended effect.
4. They can change how the kidneys remove medicines
The kidneys help remove medicines and their breakdown products from the body. Herbal products that affect kidney function, body fluids or drug transport may change how quickly a prescription medicine is removed.
This may be especially important for people who already have kidney disease or take medicines that can affect the kidneys.
5. They can add to the medicine’s effect
Sometimes an herb and a medicine produce similar effects. When taken together, the combined effect may become too strong.
For example:
- A sedating herb taken with a sedative medicine may cause excessive drowsiness.
- A herb that lowers blood sugar taken with diabetes medicine may contribute to blood sugar becoming too low.
- A product that affects blood clotting taken with a blood thinner may increase bleeding risk.
- A herb that lowers blood pressure taken with blood pressure medicine may contribute to dizziness or fainting.
This type of interaction is sometimes called an additive effect.
6. They can oppose the medicine’s effect
An herbal product may work against the intended purpose of a prescription medicine. It could raise blood pressure in someone taking blood pressure medicine or reduce the effectiveness of an immune-suppressing medicine.
In such cases, the person may think that the prescription treatment has stopped working when an interacting product is partly responsible.
7. They can increase strain on the liver or kidneys
Some herbal products can injure the liver or kidneys. Combining one of these products with a prescription medicine that affects the same organ may increase the risk of damage.
The NHS Specialist Pharmacy Service (2025) warns that kava can cause liver toxicity and may interfere with the processing of medicines such as paracetamol and statins. Aristolochia species contain aristolochic acid, which is linked to kidney damage and cancer and is prohibited or restricted in a number of jurisdictions.
Examples of Important Herbal Medicine Interactions
The following examples do not cover every possible interaction. A pharmacist or doctor should assess the exact herb, product, dose and prescription medicine.
St. John’s Wort
St. John’s wort is promoted for low mood and depression. It is one of the best-documented examples of an herb that interacts with prescription medicines.
It can speed up the body’s processing of certain medicines, lowering their concentrations and reducing their effects. Documented or clinically significant interactions include:
- Cyclosporine and other transplant medicines: Reduced drug levels can raise the risk of organ rejection.
- Some HIV medicines: Reduced concentrations may lead to treatment failure or drug resistance.
- Warfarin: The blood-thinning effect may be reduced.
- Digoxin: Blood levels and effectiveness may decrease.
- Some cancer medicines: Treatment levels may become too low.
- Some statins: Their cholesterol-lowering effect may be reduced.
- Oral contraceptives: Reduced effectiveness and unintended pregnancy may be possible.
- Some sedatives and other medicines: Drug levels may change (NCCIH, 2024, 2026).
St. John’s wort can also interact with antidepressants in a different way. Combining it with medicines that increase serotonin may raise the risk of serotonin syndrome. This potentially serious condition can cause agitation, confusion, sweating, diarrhoea, tremor, muscle stiffness, fever and a rapid heartbeat (NCCIH, 2026).
A person should not start or stop St. John’s wort without discussing it with a healthcare professional. Stopping it can also change the levels of an interacting prescription medicine as the affected enzymes return towards their previous activity.
Ginkgo Biloba
Ginkgo is promoted for memory, circulation and other purposes. Evidence reviewed by NCCIH indicates that taking ginkgo with warfarin has been associated with a greater risk of major bleeding than taking warfarin alone. Ginkgo may also create concerns when combined with antiplatelet medicines or other anticoagulants (NCCIH, 2024).
Possible warning signs include:
- Unusual bruising
- Frequent nosebleeds
- Bleeding gums
- Blood in urine
- Black or bloody stools
- Vomiting blood
- A severe, unusual headache
- Weakness or fainting
Some proposed ginkgo interactions remain uncertain. The risk depends on the exact product, dose, other medicines and the individual’s medical condition.
Garlic Supplements
Garlic used as food is generally different from concentrated garlic supplements. Garlic supplements may increase bleeding risk, especially when taken with anticoagulants, antiplatelet medicines or aspirin.
This is particularly important before surgery or a dental procedure. People taking garlic supplements should tell their surgeon, dentist, doctor or pharmacist. They should not stop prescribed blood-thinning medicine or change its dose without medical direction (NCCIH, 2025).
Goldenseal
Goldenseal can alter the way the body processes medicines. A clinical study described by NCCIH found that taking goldenseal extract with metformin reduced metformin levels by approximately 25% in healthy adults. Such a decrease could potentially interfere with glucose control in a person taking metformin for type 2 diabetes (NCCIH, 2024, 2026).
Goldenseal may also affect enzymes involved in processing other medicines. Therefore, the risk is not necessarily limited to metformin.
Concentrated Green Tea Products
Drinking green tea as a beverage is not always equivalent to taking a concentrated green tea extract.
High doses of green tea have been shown to reduce blood levels of nadolol, a beta-blocker used for high blood pressure and heart conditions. Concentrated green tea extracts may also reduce levels of atorvastatin and interact with other medicines (NCCIH, 2024, 2026).
People taking heart medicines should ask a pharmacist or doctor before using concentrated green tea products.
Ginseng
Asian ginseng may interact with medicines for high blood pressure, statins and some antidepressants. Research concerning its interaction with warfarin has produced mixed results. This means that a clinically important interaction has not been established in every situation, but caution and professional review remain appropriate (NCCIH, 2024).
Ginseng products may also differ considerably in composition, which makes it difficult to apply the result of one study to every product.
Valerian and Other Sedating Herbs
Valerian is promoted for sleep and anxiety. It may increase drowsiness when combined with medicines that also cause sedation. The NHS Specialist Pharmacy Service (2025) notes that amitriptyline may add to the sedating effects of valerian.
Other medicines of concern may include:
- Sleeping tablets
- Some anti-anxiety medicines
- Opioid pain medicines
- Sedating antihistamines
- Some antipsychotic medicines
- Certain antidepressants
- Anaesthetic medicines
- Alcohol
Excessive sedation can impair coordination, judgement, breathing and the ability to drive safely.
Herbal Products That Affect Blood Sugar
Some herbs and supplements may lower blood sugar. Combining them with insulin or diabetes tablets could theoretically or actually strengthen the glucose-lowering effect.
Possible symptoms of low blood sugar include:
- Shaking
- Sweating
- Hunger
- Dizziness
- Weakness
- Irritability
- Confusion
- Blurred vision
- Loss of consciousness or seizures in severe cases
However, evidence varies between herbal products. A person should not assume that an herb can safely replace prescribed diabetes treatment.
Herbal Products That Affect Blood Pressure
Some herbal products may lower blood pressure, while others may raise it. Combining them with blood pressure medicines may cause treatment failure or an excessive fall in blood pressure.
Possible symptoms of blood pressure becoming too low include dizziness, blurred vision, weakness and fainting. High blood pressure may produce no symptoms, so a person should not judge the effectiveness of treatment only by how they feel.
Herbal Products and Antidepressants
Certain herbal products may affect serotonin or add to the effects of antidepressants. St. John’s wort is the clearest example.
Serotonin syndrome is uncommon but potentially life-threatening. Symptoms may include:
- Restlessness or agitation
- Confusion
- Sweating
- Diarrhoea
- Tremor
- Muscle twitching or stiffness
- Fever
- Rapid heartbeat
- Seizures
Urgent medical help is needed when serious serotonin-syndrome symptoms are suspected.
Why Are Herbal Interactions Difficult to Predict?
Herbal medicine interactions are often harder to study and predict than interactions between standardised prescription medicines.
The ingredients may vary
The amount of an active substance can vary according to:
- The plant species
- The part of the plant used
- Where and how it was grown
- Harvesting and storage methods
- Extraction and manufacturing methods
- Whether the product is a tea, powder, oil or concentrated extract
- The recommended dose
- Other ingredients in the product
Two products carrying the same common herbal name may not necessarily contain the same chemical amounts.
Products may contain several herbs
Some traditional or commercial products contain many different plants. Each ingredient may have its own effects and interactions. The complete mixture may not have been studied.
A label that says “proprietary blend” may not clearly state the amount of each ingredient, making the risk even more difficult to assess.
The product may be contaminated or adulterated
Poor-quality products may contain heavy metals, pesticides, microorganisms or undeclared pharmaceutical substances. The NHS Specialist Pharmacy Service (2025) reports that some complementary products have been found to contain undisclosed steroids, anti-inflammatory medicines or sildenafil.
If a product contains an undeclared drug, neither the patient nor the clinician can accurately predict its effects or interactions.
Research is limited
Many proposed herb–drug interactions are based on laboratory experiments, animal studies, theoretical mechanisms or individual reports rather than strong human clinical trials. NCCIH (2024) states that well-designed clinical studies of herbal supplement–drug interactions remain limited and sometimes inconclusive.
This creates two important cautions:
- A suspected interaction is not automatically proven.
- An absence of published evidence does not prove that a combination is safe.
Who Is Most at Risk?
Anyone taking herbal and prescription medicines together may experience an interaction, but some people require particular caution.
Risk may be greater among:
- Older adults
- Children
- Pregnant or breastfeeding people
- People taking several medicines
- People with liver or kidney disease
- People preparing for surgery
- People with bleeding disorders
- Organ-transplant recipients
- People receiving cancer treatment
- People taking HIV medicines
- People with epilepsy
- People taking medicines with a narrow therapeutic index
A narrow therapeutic index means that the difference between an effective dose and a harmful or ineffective dose is small. Even a modest change in the amount of medicine in the blood can create serious problems.
Examples include:
- Warfarin
- Digoxin
- Cyclosporine
- Phenytoin
- Some cancer medicines
NCCIH (2024, 2026) specifically advises people using medicines with a narrow therapeutic index to inform their healthcare professionals about every herbal supplement they take.
Can You Separate the Doses to Prevent an Interaction?
Taking the herbal medicine and prescription medicine several hours apart may prevent certain interactions that occur inside the digestive system. However, it will not prevent every interaction.
Separating the doses will not necessarily stop an herb from:
- Changing liver-enzyme activity
- Affecting drug transporters
- Increasing bleeding risk
- Adding to sedation
- Changing blood pressure or blood sugar
- Damaging the liver or kidneys
Some enzyme effects may continue for days after an herbal product is stopped. Therefore, a person should not rely on dose separation without professional advice.
Should You Stop the Herbal Product Immediately?
Not always. The correct action depends on the product, prescription medicine, medical condition and seriousness of the possible interaction.
Some herbal products can cause withdrawal symptoms or changes in the levels of interacting medicines when they are stopped. The NHS Specialist Pharmacy Service (2025), for example, advises that St. John’s wort may need to be reduced gradually in some situations.
Do not stop a prescribed medicine to continue an herbal remedy. Speak to a doctor or pharmacist about the safest plan. If serious symptoms occur, seek urgent medical attention.
Herbal Medicines and Surgery
Herbal products can interfere with surgery by affecting:
- Blood clotting
- Blood pressure
- Heart rate
- Blood sugar
- Sedation
- Anaesthetic medicines
- Liver metabolism
The FDA (n.d.) advises people preparing for surgery to discuss every supplement they use with their healthcare team. A clinician may recommend stopping certain products before the procedure. The exact timing should be decided by the surgical or anaesthetic team rather than guessed by the patient.
A patient should also tell the dentist before an extraction or another procedure that may cause bleeding.
Herbal Medicines During Pregnancy and Breastfeeding
Pregnancy changes how the body processes many substances. An herbal product may affect the pregnant person, the pregnancy or the developing baby. During breastfeeding, substances from an herbal product may pass into breast milk.
Many herbs have not been studied adequately during pregnancy or breastfeeding. “Traditional use” does not prove safety for a developing baby or nursing infant.
Pregnant or breastfeeding people should consult an appropriately qualified healthcare professional before using herbal products. The FDA (n.d.) also advises discussing all supplements during pregnancy and breastfeeding.
How to Use Herbal Products More Safely
Tell healthcare professionals everything you take
Give your doctor and pharmacist a complete list containing:
- Prescription medicines
- Over-the-counter medicines
- Herbal medicines
- Traditional remedies
- Vitamins and minerals
- Dietary supplements
- Energy or performance products
- Recreational substances
Include the product name, ingredients, dose and frequency. If possible, bring the original container or clear photographs of the front label, ingredient list and dosage instructions.
Do not describe an herbal product only as “some herbs” or “a tonic.” The exact ingredients matter.
Ask before starting a new product
A pharmacist or doctor can check whether the product may interact with current treatment. This is especially important for blood thinners, transplant medicines, cancer treatment, HIV medicines, seizure medicines, diabetes treatment and heart medicines.
Use a reputable product
Choose products from trustworthy sources with clear ingredient lists, batch details, expiry dates and appropriate quality controls. This cannot eliminate every risk, but it may reduce the likelihood of contamination, adulteration or inaccurate labelling.
Do not exceed the recommended dose
Taking more of a product does not necessarily produce greater benefit. A higher dose may increase toxicity or interaction risk.
Avoid frequently changing brands
Products sold under the same herbal name may vary in strength and composition. Changing brands can expose a person to a different amount or mixture of active substances.
Monitor for changes
After starting, stopping or changing an herbal product, watch for:
- New symptoms
- Worsening of the original illness
- Unexpected side effects
- Changes in blood pressure
- Changes in blood sugar
- Unusual bleeding or bruising
- Excessive drowsiness
- Reduced effectiveness of prescription treatment
Some people may require blood tests or other monitoring.
When Should You Seek Urgent Medical Help?
Seek emergency medical attention if a possible interaction causes:
- Difficulty breathing
- Swelling of the lips, tongue, face or throat
- Fainting or loss of consciousness
- A seizure
- Severe confusion
- Chest pain
- A very rapid or irregular heartbeat
- Severe weakness or paralysis
- Vomiting blood
- Black or bloody stools
- Heavy or uncontrolled bleeding
- A sudden severe headache
- Yellow skin or eyes
- Very dark urine with severe weakness or abdominal pain
- Severe agitation, fever, muscle stiffness or tremor
- Signs of dangerously low blood sugar that do not improve promptly
Take the herbal product container and a list of all medicines to the healthcare facility if it is safe to do so.
Questions to Ask a Doctor or Pharmacist
Before using an herbal product, consider asking:
- Could this product interact with any of my medicines?
- Is the evidence for its claimed benefit reliable?
- Is the product safe with my medical conditions?
- Could it affect my liver, kidneys, blood pressure or blood sugar?
- Will I need additional monitoring?
- Should I stop it before surgery or dental treatment?
- What symptoms should make me stop using it or seek help?
- If I stop it, should I do so gradually?
- Is the food form different from the concentrated supplement?
- How can I identify a reliable product?
The Bottom Line
Herbal medicines can interact with prescription medicines. They may change how a medicine is absorbed, processed or removed from the body. They may also strengthen the medicine’s effects, oppose its action or add new risks such as bleeding, excessive sedation, low blood sugar, liver damage or treatment failure.
Some interactions—especially those involving St. John’s wort—are well documented. Other warnings are based on limited or conflicting evidence. This uncertainty is not proof of safety. It means the exact risk may not yet be known.
People should not hide their use of traditional or herbal remedies from healthcare professionals. Doctors and pharmacists need this information to protect the patient, not to judge personal or cultural health practices. The safest approach is to keep a complete medicine list, ask before starting a new herbal product and never stop an important prescription medicine without professional guidance.
Disclaimer
This article is provided for general educational purposes only. It does not replace diagnosis, treatment or individual advice from a qualified healthcare professional. Herbal products and prescription medicines can interact differently depending on the exact ingredients, doses, medical conditions and other treatments involved. Do not start, stop or change any herbal or prescription medicine based solely on this article. Consult a doctor or pharmacist for advice about your specific products. Readers are encouraged to consult the original sources. For factual errors or necessary corrections, please contact us.
References
National Center for Complementary and Integrative Health. (2024, December). Herb-drug interactions. U.S. Department of Health and Human Services, National Institutes of Health. https://www.nccih.nih.gov/health/providers/digest/herb-drug-interactions
National Center for Complementary and Integrative Health. (2025, February). Garlic: Usefulness and safety. U.S. Department of Health and Human Services, National Institutes of Health. https://www.nccih.nih.gov/health/garlic
National Center for Complementary and Integrative Health. (2026). 6 tips: How herbs can interact with medicines. U.S. Department of Health and Human Services, National Institutes of Health. https://www.nccih.nih.gov/health/tips/tips-how-herbs-can-interact-with-medicines
National Center for Complementary and Integrative Health. (2026). How medications and supplements can interact. U.S. Department of Health and Human Services, National Institutes of Health. https://www.nccih.nih.gov/health/know-science/how-medications-and-supplements-can-interact/some-supplements-may-decrease-the-effects-of-medications
Specialist Pharmacy Service. (2025, December 1). Advising on complementary products and conventional medicines. National Health Service. https://sps.nhs.uk/articles/advising-on-complementary-products-and-conventional-medicines/
U.S. Food and Drug Administration. (n.d.). Mixing medications and dietary supplements can endanger your health. https://www.fda.gov/consumers/consumer-updates/mixing-medications-and-dietary-supplements-can-endanger-your-health
