Which Herbs Should Not Be Combined? Herb-Drug Interactions – A Comprehensive Guide

Short answer: Herbs are not "delicate teas," but blends of bioactive compounds that interact in real pharmacokinetic (via CYP450 enzymes) and pharmacodynamic (at the receptor level) ways. The most problematic St. John's wort (CYP3A4 inducer lowering the concentration of several dozen drugs), anti-aggregation herbs combined with warfarin (ginkgo, garlic, ginger, ginseng), sedative herbs combined with benzodiazepines and alcohol (valerian, lemon balm, hops) and CBD, which, as a CYP3A4/2C19 inhibitor, increases concentrations of clobazam, warfarin, and tacrolimus. The rule of thumb is: before reaching for the herb, show your medication list to your doctor or pharmacist and check for interactions in a database like Lexicomp or UpToDate.

This guide is based on Polish and international sources and the daily practice of a cannabis store. It is not medical advice., but solely as educational material. Always make therapeutic decisions in consultation with your doctor.

Why do herbs interact with medications?

According to meta-analysis Izzo & Ernst (2009) published in the journal Drugs, over 100 individual herbs have documented interactions with at least one prescription drug. There are two mechanisms: pharmacokinetic (altering drug absorption, metabolism, or excretion) and pharmacodynamic (additive effects on the same receptor or system).

The vast majority of problems take place in liver, at the level of cytochrome P450, especially isoforms CYP3A4 (metabolizes approximately 50% drugs on the market), CYP2D6, CYP2C9 i CYP2C19. The herb can work in two ways: induce an enzyme (like St. John's wort, which accelerates the breakdown of drugs and lowers their concentration) or inhibit go (like CBD or grapefruit, which block metabolism and increase concentrations).

Three levels of risk worth distinguishing

Firstly, theoretical interaction, described in in vitro studies but rarely observed clinically. Secondly, interaction documented in case reports, where specific patients experienced complications. Third, interaction confirmed in clinical trials, where changes in concentrations or pharmacological effect were statistically significant. The strongest evidence is for St. John's wort, ginkgo, and CBD.

Why doesn't "natural" mean "safe"?

Foxglove produces digoxin, hellebore causes cardiac arrest, and belladonna kills. They are all natural. The word "herbal" indicates only the origin, not the potency. Dasgupta (2003) in a review published in Therapeutic Drug Monitoring points out that herbal supplements can falsify drug concentration test results, leading to incorrect clinical decisions.

St. John's wort, the most dangerous herb in the Polish medicine cabinet

St. John's wort (Hypericum perforatum) is a classic inducer of CYP3A4, P-glycoprotein and partially CYP2C9. According to a classic study Henderson et al. (2002), published in British Journal of Clinical Pharmacology, St. John's wort reduces the concentrations of key drugs by 30-70%, in some cases leading to a complete loss of therapeutic effect.

What medications does St. John’s wort interact with the worst?

• Warfarin and acenocoumarol: decrease in INR, risk of thrombosis, stroke, pulmonary embolism.
• Hormonal contraception: documented cases of unplanned pregnancies with the simultaneous use of the pill and St. John's wort.
• Statins (atorvastatin, simvastatin): decrease in cholesterol lowering effectiveness.
• Immunosuppressants (cyclosporine, tacrolimus): risk of graft rejection (described cases in transplantology).
• HIV medications (protease inhibitors): loss of virological control.
• SSRI antidepressants (sertraline, fluoxetine, paroxetine): paradoxically, both the decrease in effectiveness and the risk serotonin syndrome, because St. John's wort itself has a serotonergic effect.
• Digoxin: decrease in concentration, risk of heart failure.
• Chemotherapeutics (irinotecan, imatinib): loss of antitumor effect.

What do patients say about the pharmacy?

In our practice, several people come into our store every month to buy St. John's wort and ask for "something to help with sleep." Only then does it become clear that they're also taking SSRIs or warfarin. The level of ignorance is enormous: most customers consider St. John's wort a "calming herb," not a pharmacologically active herbal medicine that alters liver metabolism.

Quote capsule: John's wort (Hypericum perforatum) as an inducer of the CYP3A4 enzyme reduces the concentrations of warfarin, hormonal contraceptives, statins, cyclosporine and many other drugs by 30-70% (Henderson et al., 2002). Patients on chronic medications should avoid St. John's wort preparations or consult their inclusion with their physician.

Ginkgo Biloba and the Risk of Bleeding

Ginkgo biloba inhibits platelet aggregation by antagonizing platelet-activating factor (PAF). According to a review Izzo & Ernst (2009) Several cases of serious bleeding (intracranial, into the vitreous humor, postoperative) have been described in patients combining ginkgo with aspirin, warfarin or in the perioperative period.

The most dangerous combinations with ginkgo

• Ginkgo + warfarin: increased risk of bleeding, including intracranial bleeding.
• Ginkgo + aspirin or clopidogrel: cumulative anti-aggregation effect.
• Ginkgo + NSAIDs (ibuprofen, naproxen): risk of gastrointestinal bleeding.
• Ginkgo + antiepileptic drugs (valproic acid, phenytoin): some descriptions of increased risk of seizures.
• Ginkgo before planned surgery: the standard recommends weaning at least 14 days before surgery.

Wild garlic, ginger and other anti-aggregation "kitchen" herbs

According to Dasgupta (2003), combining blood-thinning herbs with warfarin changes the INR by an average of 0.4–1.2 points, which is enough to cause clinically significant bleeding in some patients. The risk increases exponentially when herbs are cumulative.

The Classic Trap: Three Anti-Aggregation Herbs at Once

The patient is admitted in the morning warfarin (cardiologist's recommendation), for breakfast a sandwich with wild garlic (seasonal treatment), for tea fresh ginger („for immunity”), and in the evening a capsule ginkgo ("by heart"). Each of these components individually has an antiplatelet effect. Together, they can increase the risk of bleeding by 100-200% compared to warfarin alone.

Other herbs that affect clotting

• Ginseng (Panax ginseng): both decreased and increased INR cases; caution recommended in patients on warfarin.
• Evening primrose and borage: gamma-linolenic acid may inhibit platelet aggregation.
• Turmeric in high doses: anti-aggregation effect.
• Chamomile (Matricaria recutita): slight anticoagulant effect; a single case of bleeding was described in a patient on warfarin who drank large amounts of the infusion.

Licorice, Ephedra, and Cardiac Herbs That Should Never Be Combined with Heart Medication

Licorice (Glycyrrhiza glabra) contains glycyrrhizin, which causes sodium retention, potassium loss, and increased blood pressure. According to Izzo & Ernst (2009) As little as 100 mg of glycyrrhizin per day (equivalent to several grams of root) can cause hypokalemia, hypertension and edema in predisposed individuals.

Licorice, or non-obvious interactions

• Licorice + antihypertensive drugs: effect cancellation, rapid pressure fluctuations.
• Licorice + thiazide or loop diuretics: increased potassium loss, risk of cardiac arrhythmias.
• Licorice + digitalis glycosides (digoxin): hypokalemia increases digoxin toxicity.
• Licorice + corticosteroids: intensification of steroid effects.

Ephedrine and caffeine, a real heart bomb

Connection ephedrine (from bedstraw, ma huang or some "fat burners") with caffeine in energy drinks has historically been implicated in cardiac deaths, including known cases in athletes. This combination should never be used, especially in patients with concomitant hypertension, arrhythmias, or coronary artery disease.

Sedative herbs and benzodiazepines, or the risk of excessive sedation

According to the review Izzo & Ernst (2009), Combining herbs that act on GABA receptors with benzodiazepines, hypnotics (zolpidem, zopiclone), or alcohol can cause excessive sedation, respiratory distress, and falls, especially in seniors. The risk is real, although it is less likely to be fatal than interactions with warfarin.

The most strongly sedative herbs

• Valerian (Valeriana officinalis): GABA-A receptor agonist, enhances the effect of benzodiazepines (alprazolam, lorazepam, diazepam, clonazepam) and non-benzodiazepine hypnotics.
• Lemon balm (Melissa officinalis): cumulative sedative effect, risk of excessive drowsiness in combination with benzodiazepines and first-generation antihistamines.
• Hop (Humulus lupulus): enhances the effects of valerian and alcohol.
• Passion flower (Passiflora incarnata): increases the sedative effect of MAOIs and benzodiazepines.
• Lavender (essential oils and capsules): described cases of increased sedation in children and seniors.

When does a „gentle calming tea” become a problem?

A patient taking alprazolam for anxiety in the evening adds a valerian infusion "for better sleep" and a glass of wine. The three GABAergic substances work synergistically. In the morning: lightheadedness, difficulty concentrating at work, and risk of falling in an elderly person. This isn't a hypothetical scenario, but rather a daily practice in geriatric wards.

Echinacea, ginseng, and immunomodulatory herbs

Echinacea (purple coneflower) is the most frequently purchased "immunity-boosting" herb in Polish pharmacies, but according to Izzo & Ernst (2009) may reduce the effectiveness of immunosuppressive drugs in transplant patients and in patients with autoimmune diseases. Modulation of the immune system is not always desirable.

When are echinacea and ginseng not recommended?

• Echinacea + cyclosporine or tacrolimus: in transplant recipients – risk of loss of control of immunosuppression.
• Echinacea + methotrexate: in patients with rheumatoid arthritis – risk of exacerbations.
• Echinacea + chronic autoimmune diseases: multiple sclerosis, lupus, psoriasis – contraindicated.
• Ginseng + warfarin: INR fluctuations described.
• Ginseng + MAOI (e.g. phenelzine, moclobemide): hypertension, headache, tremors.
• Ginseng + antidiabetic drugs: risk of hypoglycemia.

CBD, or the growing problem of interaction in Polish homes

According to Brown & Winterstein (2019), published in Journal of Clinical Medicine, CBD is a clinically significant inhibitor of CYP3A4 and CYP2C19 and, to a lesser extent, CYP2D6 and CYP1A2. This means it can increase concentrations of a whole class of drugs metabolized by these pathways, sometimes by as much as 70-80%.

What medications does CBD actually interact with?

• Clobazam: pediatric doses in patients with drug-resistant epilepsy; CBD increases the concentration of the active metabolite N-desmethylclobazam up to threefold.
• Warfarin: described cases of significant increase in INR and bleeding.
• Tacrolimus and cyclosporine: risk of nephrotoxicity.
• SSRI/SNRI antidepressants metabolized by CYP2C19: citalopram, escitalopram, sertraline.
• Valproates: risk of increased liver transaminases.
• Proton pump inhibitors (omeprazole, esomeprazole): change in concentrations.
• Statins metabolized by CYP3A4: simvastatin, atorvastatin – risk of myopathy.

CBD + grapefruit, a synergistic effect

Grapefruit juice inhibits the same CYP3A4 enzyme. Combining CBD with grapefruit is double metabolism blockade Many medications, which increases the risk of side effects. The same rule applies to pomelo juice and bitter oranges. If a drug package insert warns against grapefruit, caution also applies to CBD.

THC, alcohol and benzodiazepines – combination prohibited

THC intensifies the effects of alcohol, benzodiazepines and sedative drugs. WHO ECDD Critical Review of CBD (2018) clearly indicates that CBD itself has a "favorable safety profile" and does not show addictive potential, but pharmacokinetic interactions require medical supervision, especially for drugs with a narrow therapeutic window.

Quote capsule: CBD as an inhibitor of CYP3A4 and CYP2C19 enzymes may significantly increase the concentrations of warfarin, clobazam, tacrolimus and some SSRIs, generating the risk of side effects (Brown & Winterstein, 2019). Patients on chronic medications should start CBD under the supervision of a physician and monitor their concentrations and INR.

What herb-to-herb combinations should you avoid?

According to Izzo & Ernst (2009), approximately 40% of adverse events attributed to herbs involve combinations of multiple plants in OTC preparations and home remedies. The sum of pharmacodynamic effects is the most common mechanism of harm.

The most common problematic herbal mixtures

• St. John's wort + lemon balm + valerian: paradoxically found in "nerve" mixtures; St. John's wort has a serotonergic effect, lemon balm and valerian have a sedative effect - risk of excessive sedation during the day and agitation at night.
• Ginkgo + garlic + ginger: three anti-aggregation herbs – cumulative effect, risk of bruising, nosebleeds, menstrual bleeding.
• Echinacea + St. John's wort: double immune and metabolic modulation; the case of Mrs. Anna described in the previous version of the article is a classic warning.
• Ma huang (ephedrine) + guarana or coffee: cardiological bomb.
• Licorice + diuretic herbs (horsetail, birch): potassium loss plus sodium retention; electrolyte chaos.
• Three or more sedative herbs at once: valerian, lemon balm, hops, passionflower, lavender – unpredictable effect.

Grandma's mixtures are not safer than medicines

We often hear, "My grandma's been doing this for 40 years." Anecdotal evidence isn't a substitute for pharmacological evidence. Grandma might have had a liver with a different CYP3A4 activity, wasn't taking statins or SSRIs, and couldn't remember the times when "something happened to her after tea." Selective memory bias is a weak argument when you're currently taking five prescription medications.

Who is at increased risk?

According to Dasgupta (2003), up to 70% serious complications following herbal preparations affect patients from four groups: pregnant women, seniors, oncology patients, and people on chronic medications. These groups should only use herbs after consulting a doctor.

Pregnant and breastfeeding women

Many herbs have an abortive effect (yarrow in large doses, rue, spearmint, parsley in oil), estrogenic effects (hops, soy, red clover) or pass into milk (chamomile, dill, mint - they can affect lactation). The Golden Rule: during pregnancy and breastfeeding no herbs without the consent of a gynecologist.

Seniors

Polypharmacy (taking 5 or more medications), altered hepatic metabolism, reduced renal function, and increased CNS sensitivity place seniors at greatest risk for complications. Sedation after valerian in a 30-year-old is uncomfortable. In an 80-year-old, it can result in a fall, hip fracture, and hospitalization.

Oncology patients

Chemotherapeutic agents have a narrow therapeutic window. St. John's wort may reduce irinotecan concentrations by 40%. Echinacea, which modulates the immune system, may interfere with immunotherapy. Any herb prescribed to a cancer patient must be discussed with the treating oncologist..

People on chronic medications

Warfarin, statins, SSRIs, hormonal contraception, immunosuppressants, antidiabetic drugs, antipsychotics, cardiological drugs – these are groups in which even „safe” chamomile or lemon balm requires checking in the interaction database.

List of drugs requiring special caution

According to Office for Registration of Medicinal Products, most reports of adverse herbal-drug interactions in Poland concern the list below. If you are taking any of these medications, consult each new herb individually.

Drugs with a narrow therapeutic window

• Warfarin and acenocoumarol (anticoagulant).
• Clobazam (drug-resistant epilepsy – especially with CBD).
• Tacrolimus and cyclosporine (after transplants).
• Digoxin (heart failure, atrial fibrillation).
• Lithium (bipolar affective disorder).
• Phenytoin, carbamazepine (epilepsy).

Psychiatric medications

• SSRIs/SNRIs: sertraline, fluoxetine, paroxetine, escitalopram, venlafaxine.
• MAOI: phenelzine, moclobemide, tranylcypromine.
• Tricyclic: amitriptyline, clomipramine, nortriptyline.
• Benzodiazepines: alprazolam, lorazepam, diazepam, clonazepam.
• Antipsychotics: quetiapine, olanzapine, risperidone, clozapine.

Cardiological, hormonal and oncological drugs

• Statins: simvastatin, atorvastatin, rosuvastatin.
• Beta-blockers and calcium channel blockers.
• Hormonal contraception and HRT.
• Antidiabetic drugs: metformin, gliclazide, insulin.
• Chemotherapeutics and targeted therapies: irinotecan, imatinib, paclitaxel.
• Proton pump inhibitors: omeprazole, esomeprazole, pantoprazole.

How to Use Herbs Safely? Five Rules

According to the guidelines Izzo & Ernst (2009), over 80% adverse herbal events could have been avoided by following five simple rules. These rules are free and work better than any "safe" supplement.

1. List of all medications – for your doctor and pharmacist

Before every doctor's appointment, make an up-to-date list: prescription medications, OTC medications, supplements, vitamins, herbs, and CBD. The doctor won't guess that you're drinking valerian tea in the evenings. The pharmacy won't know either. Be upfront about it.

2. One herb at a time for several weeks

Introducing a single herb allows you to observe the reaction and attribute any symptoms to a specific plant. Simultaneously incorporating five herbs prevents even a doctor from identifying the culprit of the side effects.

3. Interaction databases: Lexicomp, UpToDate, Drugs.com

Professionals use the Lexicomp, UpToDate, and Micromedex databases. Patients can check the free interaction module on Drugs.com. If you type "warfarin + ginkgo biloba," you'll get a "major interaction" rating with a description of the mechanism. It takes just a few seconds.

4. A doctor's prescription for every doubt

An herbal supplement isn't a substitute for a consultation. If you have any doubts about whether licorice root can help with high blood pressure, schedule an appointment. It's cheaper than hospitalization for hypokalemia.

5. Don't treat herbs as "mild teas"„

Foxglove, aconite, hellebore—all are herbs. Foxglove produces digoxin. The "natural" category doesn't include "pharmacologically inactive." Each herb has a risk profile, which is worth understanding before your first cup.

How to report an adverse reaction in Poland?

According to the data Office for Registration of Medicinal Products, Medical Devices and Biocidal Products (URPL), there is a system in Poland Department of Monitoring Adverse Reactions of Medicinal Products. Any citizen can report a suspected adverse event free of charge and anonymously.

Three ways to report

• Online form on the website urpl.gov.pl, section "Report an adverse reaction".
• Paper form sent by post or by a doctor.
• Mobile application Mobit Scanner, available on Android and iOS.

Applications are entered into the database EudraVigilance and help detect patterns of adverse events that would not be captured by isolated clinical cases. Your report is valid even if you're not sure whether the herb was actually responsible for the reaction.

Polish Legal Context – Herbs, Dietary Supplements, and CBD

In Poland, most of the herbal preparations available in the market are registered as dietary supplements supervised by the Chief Sanitary Inspectorate (GIS), and not as medicinal products. Act of 29 July 2005 on Counteracting Drug Addiction defines the status of hemp, and CBD products obtained from hemp with a THC content below 0.3% are legal in Poland.

A dietary supplement is not a drug

Supplement manufacturers are not required to prove clinical effectiveness, and quality control is less stringent than for medications. This means that the active ingredient content in an herbal preparation may differ from the declared value. Choose manufacturers who use standardization extract and COA (Certificate of Analysis) certificates from independent laboratories.

CBD in Poland

CBD oil is legally available as a dietary supplement or cosmetic, provided the THC content is below 0.31 TP3T. The market offers broad-spectrum (THC-free), full-spectrum (with THC within legal limits), and isolates (pure CBD). Regardless of the form, the same precautions apply when combining with drugs metabolized by CYP3A4 and CYP2C19.

When does CBD make sense and when doesn't it?

According to 2018 WHO ECDD Critical Review, CBD has a favorable safety profile but should not be used unsupervised in patients on medications with a narrow therapeutic window. The key is an informed choice, a proven product, and dialogue with your doctor.

What we see in the store

The most common questions from clients concern insomnia, anxiety, chronic pain, and recovery from exercise. Some of these people take SSRIs, benzodiazepines, or pain relievers. Our role is to highlight potential interactions and encourage discussion with a healthcare provider, not to replace a medical consultation.

For people who decide to start supplementation after consultation, we most often recommend starting with a low dose of broad-spectrum, e.g. SOOL CBD 5% oil as a starting variant. For more experienced users, higher concentration oils are available, such as SOOL CBD 10%, and in the field of CBG, the quality choice is Cannova CBG 15%. For those who prefer dried form – Mars CBD 9%.

FAQ – Frequently asked questions about combining herbs and drug interactions

Can I drink chamomile if I take warfarin?

Chamomile (Matricaria recutita) has a slight anticoagulant effect, a single case of bleeding was described in a patient on warfarin who drank large amounts of the infusion (Izzo & Ernst, 2009). An occasional cup is usually safe, but drinking several liters of the infusion daily requires consultation with your doctor regarding your INR.

How long before surgery should you stop taking herbs?

The surgical standard recommends discontinuing herbs that affect clotting (ginkgo, garlic, ginger, ginseng, St. John's wort) for at least 14 days before the planned procedure (Dasgupta, 2003). Always provide your anesthesiologist with a full list of medications you are taking, as some also affect the metabolism of anesthetic drugs.

Can CBD be combined with SSRI antidepressants?

CBD inhibits CYP2C19, which metabolizes citalopram, escitalopram, and sertraline, which may increase their concentrations (Brown & Winterstein, 2019). The combination is not prohibited, but requires consultation with a psychiatrist, preferably with monitoring of symptoms and possibly blood levels. Do not discontinue SSRIs on your own to start CBD.

Does St. John's wort actually reduce the effectiveness of contraceptive pills?

Yes, clinical studies and case reports confirm that St. John's wort induces CYP3A4 and reduces the concentrations of ethinylestradiol and progestogens, which has led to unplanned pregnancies (Henderson et al., 2002). Women on hormonal contraception should avoid St. John's wort or use an additional barrier method if its use is necessary.

Is lemon balm safe for children and seniors?

Lemon balm is one of the mildest sedative herbs, but in children and seniors it may cause increased sedation, especially in combination with first-generation antihistamines or benzodiazepines (Izzo & Ernst, 2009). Use moderate doses, one at a time, and monitor the reaction. If you have a child, consult a pediatrician.

Can I drink grapefruit juice with CBD oil?

This is not recommended. Both CBD and grapefruit inhibit the CYP3A4 enzyme, so the combination works synergistically and may increase concentrations of drugs metabolized by this pathway (Brown & Winterstein, 2019). If your medication leaflet warns against grapefruit, caution also applies to CBD, and vice versa.

Can herbs replace medications for high blood pressure or diabetes?

No. Herbs can support therapy, but stopping medications for high blood pressure, diabetes, epilepsy, depression, or heart disease on your own is dangerous. Dasgupta (2003) describes cases of dramatic complications following an unintentional switch from pharmacotherapy to phytotherapy. Each change requires the discretion of the treating physician.

What are the first warning signs that an herb is not working?

The most common symptoms of interactions include: nausea, dizziness, excessive sleepiness or insomnia, bleeding (gums, nose, easy bruising), blood pressure fluctuations, heart palpitations, rash, itching, diarrhea, abdominal pain, and yellowing of the skin. If these symptoms occur, discontinue use and consult a doctor. In case of an acute emergency, call 112.

Do herbal supplement manufacturers in Poland have to test for interactions?

No. Dietary supplements in Poland are regulated by the Chief Sanitary Inspectorate (GIS) and are not subject to the same requirements as medications. Manufacturers are not required to prove efficacy or conduct interaction studies. Therefore, responsibility for safe use rests partly with the patient and physician, and choosing a manufacturer with full documentation (COA, standardization) is crucial.

Where to report an adverse reaction to a herbal preparation?

Applications are accepted URPL (Office for Registration of Medicinal Products) via an online form, a paper form, or the Mobit Scanner app. Reporting is free, anonymous, and valid, even if you're unsure about the causal relationship. The data is entered into the European EudraVigilance database and helps monitor the safety of medications on the market.

Summary

Herbs are a powerful tool, but not a magical alternative to pharmaceuticals. The three most dangerous pitfalls are: St. John's wort as a CYP3A4 inducer lowering the concentrations of several dozen drugs, anti-aggregation herbs combined with warfarin and aspirin and sedative combinations with benzodiazepines and alcohol. The fourth, growing group is CBD as an inhibitor of CYP3A4 and CYP2C19, which should be treated with caution with drugs with a narrow therapeutic window.

Five safety rules: consult a doctor and pharmacist for a complete medication list, administer one herb at a time, check drug interactions in databases, obtain a prescription when in doubt, and treat herbs as pharmacologically active preparations, not "mild teas." Report any adverse reactions to the Office for Public Health (URPL). In case of an acute condition, call 112.

This article is not medical advice.. Decisions about using, combining, or discontinuing any medications or herbs should be made only with your doctor.

Author: Michał Waluk, ubucha.pl. Updated: April 25, 2026.