Which herbs should not be combined - interactions and contraindications (table)
Which herbs should not be combined: interactions, contraindications, and precautions in a clear table. u Bucha.
Over 50% of adults in EU countries take herbs or herbal supplements simultaneously with medications prescribed by their doctor, and only one in five informs their doctor about it (Izzo & Ernst, Drugs, 2009). This silence can have serious consequences. Herbs are not neutral plants: they contain active compounds that affect liver enzymes, blood clotting, and the nervous system. This article specifically shows which herbs should not be combined with each other or with medications, the mechanisms behind this, and who should be particularly cautious. At the end of each section, you will find indications of when a medical consultation is required.
KEY INFORMATION
• Over 50% of people taking herbs also use medications prescribed by their doctor (Izzo & Ernst, Drugs, 2009).
• St. John's Wort is the most common cause of clinical herbal-drug interactions in Europe - it induces CYP3A4 enzymes and weakens over 50% of prescription medications.
• Ginkgo, garlic, and ginseng in supplements enhance the effects of anticoagulants and increase the risk of bleeding.
• Valerian combined with benzodiazepines or alcohol can cause dangerous excessive sedation of the CNS.
• Licorice in high doses disrupts electrolyte balance and may weaken antihypertensive medications.
Why can herbs interact with medications?
Herbs contain hundreds of active phytochemicals that affect liver enzymes from the cytochrome P450 family. The CYP450 system metabolizes up to 60% of all medications available on the market, which is why any substance altering the activity of these enzymes directly affects the concentration of medications in the blood (PMC, 2007). Herbs can either accelerate the metabolism of a drug (the active substance is removed more quickly) or slow it down (the drug accumulates to toxic levels).
There are two main mechanisms of interaction. The first is enzyme induction: the herb increases the activity of enzymes, and the drug is metabolized too quickly - its concentration drops, and its therapeutic effect diminishes. The second is inhibition: the herb blocks enzymes, causing the drug not to be broken down quickly enough and accumulate in the body, increasing the risk of side effects.
We have noticed that most articles on herbal interactions focus solely on liver metabolism. Meanwhile, the impact on P-glycoprotein (P-gp) - a membrane transporter that removes substances from intestinal and brain cells - is equally important. St. John's Wort induces both CYP3A4 and P-gp simultaneously, which doubles its potential to lower drug concentrations. This is a rarely described but crucial mechanism.
The second important mechanism is pharmacodynamic action: the herb and the drug act on the same biological target, enhancing or weakening their effects. Valerian and benzodiazepines both inhibit the CNS - when combined, they act more strongly than would be expected from the sum of doses. This mechanism is not visible in enzymatic studies, making it easy to overlook.
Which herbs should not be combined - interaction table
The table below collects eight herbs with clinically documented interactions. The data is based on systematic reviews published in peer-reviewed scientific journals, including the key work by Izzo & Ernst (2009) covering over 300 clinical cases. The "For whom particularly important" column indicates groups requiring mandatory medical consultation.
| Herb | Do not combine with | Mechanism / risk | For whom particularly important |
|---|---|---|---|
| St. John's Wort (Hypericum perforatum) | Anticoagulants, hormonal contraception, digoxin, antiretroviral drugs (HIV), cyclosporine, SSRI antidepressants | Induction of CYP3A4 and P-glycoprotein - lowering drug concentrations; risk of serotonin syndrome with SSRIs | People post-transplant, with HIV, using hormonal contraception, on anticoagulant medications |
| Ginkgo biloba (Ginkgo biloba) | Warfarin, heparin, aspirin, clopidogrel, NSAIDs | Inhibition of platelet aggregation - additive risk of bleeding, including to the CNS | Individuals after surgical procedures, taking anticoagulants or antiplatelet drugs |
| Valerian (Valeriana officinalis) | Benzodiazepines, alcohol, sleeping pills, barbiturates, other sedatives | Additive CNS depression - excessive sedation, concentration disturbances, risk when driving | Individuals taking psychotropic or sedative medications, drivers |
| Licorice (Glycyrrhiza glabra) | Antihypertensive drugs, corticosteroids, diuretics, potassium preparations | Pseudoaldosteronism at doses above 100 mg/day - hypokalemia, sodium retention, increased blood pressure | Individuals with hypertension, heart failure, taking diuretics |
| Ginseng (Panax ginseng) | Warfarin, diabetes medications (insulin, metformin), MAO inhibitors, estrogens | Increased effect of anticoagulants; hypoglycemia with insulin; risk of hypertensive crisis with MAOIs | Diabetics, individuals on psychiatric medications, taking anticoagulants |
| Garlic in supplements (Allium sativum) | Warfarin, clopidogrel, aspirin, antiretroviral drugs (saquinavir) | Inhibition of platelet aggregation and induction of CYP3A4 - increased risk of bleeding; reduced effectiveness of HIV medications | Individuals with HIV, on anticoagulants, before surgery (discontinue 7-10 days prior) |
| Pokrzywa (Urtica dioica) | Diuretics, diabetes medications, NSAIDs, antihypertensive drugs | Additive diuretic and hypoglycemic effects - risk of dehydration, hypoglycemia, and electrolyte disturbances. | Diabetics, individuals taking diuretics or antihypertensive medications |
| Echinacea (Echinacea) | Immunosuppressive drugs (cyclosporine, tacrolimus), drugs metabolized by CYP1A2 and CYP3A4 | Stimulation of the immune system negates the effects of immunosuppressants; heterogeneous impact on CYP | Individuals post-transplant, with autoimmune diseases, on chronic immunosuppression |
Important note for the table: the risk increases with dosage and duration of use. A one-time St. John's Wort tea will not effectively lower warfarin levels. Regular intake of standardized extracts over several weeks will. Supplements contain significantly higher doses than home infusions, which is why interactions primarily concern supplementation products.
St. John's Wort and medications - why does this combination require a consultation with a doctor?
St. John's Wort is responsible for the highest number of clinically documented herb-drug interactions in Europe. Its active ingredients - hypericin and hyperforin - induce both CYP3A4 enzymes and P-glycoprotein transporters, leading to a significant acceleration of the metabolism of many drugs (PMC, 2007). The result: the concentration of the drug in the blood decreases, and the therapeutic effect weakens or disappears.
The consequences can be serious. In individuals taking cyclosporine after kidney or heart transplantation, St. John's Wort can trigger episodes of acute transplant rejection. In the case of antiretroviral drugs (e.g., indinavir), the drug level in the blood can drop by more than 50%, risking HIV drug resistance. With hormonal contraception, the mechanism is the same: breakthrough bleeding is a signal that contraceptive protection is insufficient.
A separate issue is the risk of serotonin syndrome when combining St. John's Wort with SSRI antidepressants (e.g., fluoxetine, paroxetine). Both affect serotonergic transmission - together they can cause seizures, fever, and confusion. This is a condition requiring medical intervention. With SSRIs, the use of St. John's Wort is absolutely contraindicated without a doctor's supervision.
Ginkgo biloba, garlic, and ginseng - herbs affecting blood clotting.
Ginkgo biloba, garlic in supplements, and Panax ginseng share one common problem: they all inhibit platelet aggregation and can enhance the effects of anticoagulant drugs. The risk of bleeding when combining ginkgo with warfarin has been documented in a series of clinical cases published in Journal of the American Medical Association (Izzo & Ernst, Drugs, 2009). This applies even to supplemental doses, not just high ones.
Supplemented garlic (not culinary) additionally induces CYP3A4, leading to lower levels of antiretroviral drugs in HIV patients. This is an important distinction: garlic in cooking rarely reaches doses that cause clinical interactions. Garlic supplements standardized for allicin or ajoene can contain significantly higher concentrations of active compounds than a clove of garlic in soup.
Panax ginseng has an additional mechanism: it can enhance the effects of hypoglycemic drugs and insulin, leading to hypoglycemia. At the same time, it contains phytoestrogens, which can interfere with hormone replacement therapy or contraception. Individuals with diabetes, hormonal disorders, or on anticoagulants should use ginseng only after consulting a doctor.
Our observations indicate that questions about combinations of herbs and medications mainly arise before traveling or during seasonal immune supplementation. Many customers are unaware that ginkgo biloba, popular 'for memory,' and aspirin taken prophylactically is a combination that increases the risk of bleeding. It's worth checking the composition of the entire supplementation before traveling, not just during a doctor's visit.
When does a combination of herbs require mandatory consultation with a doctor?
Not every herb requires a doctor's visit before use. The problem escalates with high-dose supplements used chronically in individuals taking medications. A cohort study published by NCCIH showed that the risk of clinically significant interactions primarily concerns individuals taking more than two prescription medications simultaneously (NIH/NCCIH).
A medical consultation is mandatory in the following situations. First, when you are taking medications with a narrow therapeutic index: warfarin, digoxin, lithium, immunosuppressive drugs, antiretrovirals, or anticonvulsants. Even small changes in the concentration of these drugs can have serious clinical consequences. Second, when planning supplementation before a surgical procedure - most guidelines recommend discontinuing herbal supplements at least 7-14 days before the planned surgery.
Third, when you are pregnant or breastfeeding. Fourth, when you have a chronic illness: heart disease, diabetes, hypertension, autoimmune diseases, or coagulation disorders. In these cases, self-supplementation with herbs without a doctor's knowledge carries real risks. A pharmacist at the pharmacy can also check for basic interactions when dispensing medication.
Frequently Asked Questions
Which herbs should absolutely not be combined with medications?
Common St. John's Wort is the herb with the highest number of documented interactions with medications. It induces CYP3A4 enzymes and P-glycoprotein, which lowers the blood concentration of anticoagulants, hormonal contraception, immunosuppressive, and antiretroviral drugs (Izzo & Ernst, Drugs, 2009). Ginkgo and garlic in supplements enhance the effects of warfarin and increase the risk of bleeding.
Can valerian interact with medications?
Yes. Valerian has an additive effect that inhibits the central nervous system and enhances the effects of benzodiazepines, alcohol, and sleeping pills. This combination can lead to excessive sedation and concentration disturbances. Individuals taking psychotropic medications should consult their doctor before using valerian. The use of valerian by drivers requires caution.
Does St. John's Wort reduce the effectiveness of hormonal contraception?
Yes. St. John's Wort induces cytochrome P450 enzymes (mainly CYP3A4) and P-glycoprotein, which accelerates the metabolism of contraceptive hormones. Clinical studies confirm that regular intake of St. John's Wort can result in breakthrough bleeding and reduced effectiveness of hormonal contraception (Izzo & Ernst, Drugs, 2009). This is one of the best-documented herb-drug interactions.
Which herbs are dangerous when taking heart medications?
Three herbs require special caution with cardiological medications. St. John's Wort lowers the concentration of digoxin in the blood. Ginkgo biloba and garlic enhance the effects of anticoagulants (warfarin, heparin), increasing the risk of bleeding. Licorice in higher doses can cause pseudoaldosteronism and hypokalemia, which may intensify the effects of diuretics and disrupt heart function. Each of these herbs requires cardiological consultation when undergoing pharmacological treatment.
Can different herbs be combined without consulting a doctor?
Most mild herbs used in infusions (chamomile, mint, lime) do not pose dangerous interactions at typical doses. Problems arise with supplements and extracts at high concentrations, especially in individuals taking medications. If you are taking any medication chronically, always consult your planned herbal supplements with a doctor or pharmacist (NIH/NCCIH).
This article is for informational and educational purposes and does not replace consultation with a doctor. If you are pregnant, breastfeeding, taking medications, or have chronic conditions, consult the use of supplements or herbs with a specialist.
Author: Michał Waluk · Published: 2026-05-04 · Updated: 2026-05-04
