Supplements, herbs and medication: how to check a combination

Your supplement shelf and your medicine cabinet might live in different rooms. Your liver doesn't make that distinction. Anything you swallow goes through the same body, the same enzymes and the same bloodstream, which is why a herbal capsule can change how a prescription works.

Before anything else: tell your pharmacist or doctor about everything you take, supplements and herbs included, and bring the actual products. "Natural" is a description of where an ingredient comes from, not a safety category. This guide explains the handful of ways interactions happen, gives sourced examples, and shows what to say at the counter. Whether your combination is fine depends on your exact medicines, products and health, so that check belongs with a pharmacist.

"Natural" is where it comes from, not what it does

The Dutch Medicines Evaluation Board (CBG-MEB) puts it neatly: herbal products can look harmless, but they are not always harmless, and herbs can cause a medicine to be absorbed or broken down differently, which can weaken or intensify it, sometimes seriously (CBG-MEB, Kruiden, read 8 October 2026). The same page explains a point shoppers often miss: in the Netherlands a herbal medicine is approved by the CBG before sale, while a herbal supplement falls under food law and needs no such approval.

NCCIH's guidance for health professionals makes a similar point from the other side: many herbs carry real interaction risks, and some are known to affect important enzymes and transporters in the body (NCCIH, Herb-Drug Interactions: What the Science Says, December 2024). In short, if a substance is active enough to do something for you, it is active enough to do something to your medicine.

The usual ways interactions happen

Most of the interactions you'll hear about come down to a few mechanisms. This is our plain-language grouping of the sources below, not a quotation from any of them.

1. Changing how fast the body clears a medicine (the enzyme route). Many medicines are broken down by liver and gut enzymes, often called CYP enzymes (CYP3A4 is a well-known one). Some herbs and foods speed these enzymes up, so the medicine is cleared faster and may work less well. Others slow them down, so more of the medicine stays in the blood than expected. St John's wort is the textbook "speeder-upper"; NCCIH calls it a potent inducer of both cytochrome P-450 enzymes and an intestinal transporter called P-glycoprotein. Grapefruit is a "slower-downer" for a particular enzyme, intestinal CYP3A4.

2. Pushing the same system in the same direction. Two things that raise serotonin can add up. St John's wort combined with certain antidepressants can lead to a potentially life-threatening increase in serotonin (NCCIH, St. John's Wort and Depression).

3. Adding to a medicine's effect on bleeding. Taking ginkgo with warfarin is associated with a higher risk of major bleeding events than warfarin alone (NCCIH, December 2024).

4. Adding up sedation. NCCIH advises that kava should not be used together with other substances with a sedative effect, such as benzodiazepines or alcohol (NCCIH, Kava, updated April 2025), and that valerian should not be taken along with alcohol or sedatives, as a precaution, because a sleep-inducing effect is possible though not proven (NCCIH, Valerian, updated May 2025). In the Netherlands, kava has a further wrinkle: the Dutch herbal-preparations decree has an annex listing plants and fungi that herbal preparations must not contain (Article 4(2); it names kava and yohimbe, for example). A plant missing from that annex is not thereby approved: other rules, including EU novel-food law, are separate. A January 2026 draft amendment to the decree exists whose text we haven't seen, so don't treat the annex as final (checked 8 October 2026).

5. Interfering with a medicine's balance, as with vitamin K. Warfarin is balanced against vitamin K, so a change in how much vitamin K you take can matter. The NHS says foods high in vitamin K can affect warfarin, but that you may eat them with clinic guidance, and tells people not to take St John's wort or vitamin K supplements while on warfarin (NHS, Warfarin). That is a different issue from ginkgo's bleeding concern, which is why "blood-thinning supplements" is not one uniform category. Name the ingredient and let the pharmacist assess it.

Examples worth knowing

• Combination: St John's wort and the contraceptive pill; What the source says: Reduces the pill's reliability; the effect on hormones is still present for 4 weeks after stopping; Source: Apotheek.nl (KNMP), updated 26 November 2025
• Combination: St John's wort and a range of medicines; What the source says: Documented clinically significant interactions include ciclosporin, indinavir, oral contraceptives, warfarin, digoxin and benzodiazepines; Source: NCCIH, December 2024
• Combination: St John's wort and HIV antiretrovirals; What the source says: Can reduce the effectiveness of these medicines through enzyme induction; Source: EMA public statement (first issued 2000, modified 2010)
• Combination: St John's wort and antidepressants; What the source says: Risk of dangerously high serotonin; Source: NCCIH, St. John's Wort and Depression
• Combination: Grapefruit and some medicines; What the source says: Blocks intestinal CYP3A4, so more of the medicine reaches the blood; affected groups include some statins, blood pressure medicines and transplant medicines; Source: FDA, 2021
• Combination: Ginkgo and warfarin; What the source says: Associated with increased risk of major bleeding; Source: NCCIH, December 2024; NCCIH, Ginkgo, February 2025
• Combination: Kava and alcohol or sedatives; What the source says: NCCIH says not to use it with sedatives such as benzodiazepines or alcohol; Source: NCCIH, Kava
• Combination: Valerian and alcohol or sedatives; What the source says: NCCIH advises against, as a precaution, because a sleep-inducing effect is possible (though not proven); Source: NCCIH, Valerian
• Combination: Vitamin K and warfarin; What the source says: Can change how well warfarin works; Source: NHS

St John's wort, the overachiever

St John's wort deserves a closer look because it shows how wide an interaction can be. The Dutch pharmacovigilance centre Lareb reported 45 reports of side effects and interactions in a 2018 notice, including breakthrough bleeding on the pill and serious psychiatric symptoms when it was combined with antipsychotics or antidepressants, and warned that it interacts with the pill, cancer medicines, anticoagulants, heart medicines, epilepsy medicines, antipsychotics and antidepressants (Lareb, 23 January 2018). Apotheek.nl's advice is to have a pharmacist check it against your other medicines. Whether it is "a mild herbal" is the wrong question; the right one is what else it touches.

Grapefruit, the tiny exception to "it's just food"

The FDA notes that grapefruit's effect depends on the person, the medicine and how much juice is consumed, and that Seville oranges, pomelos and tangelos may behave the same way. It does not make grapefruit a universal test: it matters for some medicines and not for others, so read the leaflet or ask. Nothing in that advice means you should stop a medicine yourself. For people on warfarin, the NHS goes further and tells them not to drink grapefruit juice (NHS, Warfarin).

A note on kratom

Kratom comes up often in smartshop conversations about medication, so here is what the research and the health agencies say. This page sticks to interactions; product and dosing detail are out of scope.

• What lab work shows. A review of kratom-drug interaction research reported that kratom and its main alkaloid, mitragynine, inhibit the enzymes CYP2D6 and CYP3A in laboratory systems, and that case reports of multi-drug poisonings include a man admitted with serotonin syndrome who had taken venlafaxine and/or quetiapine with a very large amount of kratom, about 90 g a day (the cause was suspected, not proven) (Tanna et al., Drug Metabolism and Disposition, 2023). The authors call for more clinical studies.
• What the one clinical study found. In a small crossover study of 12 healthy adults, 2 g of kratom tea raised exposure to midazolam (a CYP3A test drug) by about 39% and had no significant effect on dextromethorphan (a CYP2D6 test drug) (Tanna et al., Clinical Pharmacology & Therapeutics, 2023; funded by a US NIH centre). One product, one small dose and healthy volunteers, so it cannot be stretched to every kratom product or medicine.
• What a health agency says. NCCIH lists drowsiness among reported side effects and says that kratom used with other drugs has been linked to deaths and severe adverse effects (NCCIH, Kratom, April 2022). Sedating effects are the reason pharmacists ask about any sedating medicines or alcohol: that link is our reasoning, not a finding in these sources.

On the Dutch legal position, as read on 8 October 2026: kratom is not on the Opium Act lists, and in a June 2025 ruling on a 1,100 kg import of kratom leaf powder (ECLI:NL:CBB:2025:340), the Dutch appeals tribunal held that kratom is not excluded from food law as a narcotic, and the authorities’ position that it is an unauthorised novel food was not contested on appeal. We found no ruling on retail smartshop sales, capsules, extracts or gummies. A March 2026 draft decision on the Opium Act lists exists, and we could not confirm it is unrelated to kratom. Law changes, so check current rules.

What to take to the pharmacist

Bring the products themselves, or clear photos of the labels, not a description such as "a natural focus blend", which can hide the very ingredient that matters. Here's what helps:

• Bring or say: Every medicine, including over-the-counter and anything occasional; Why it helps: The pharmacist can only check what they know about
• Bring or say: Every supplement, herbal product and fortified drink, with the full ingredient list; Why it helps: One brand can sell several formulations
• Bring or say: The amount and how often you actually take it; Why it helps: Exposure matters when judging an interaction
• Bring or say: When you started, and anything that changed recently; Why it helps: A new product can coincide with a change in how you feel
• Bring or say: Why you take it; Why it helps: Helps judge whether it is worth continuing

The CBG-MEB offers an online "Kruidencheck" for people who combine herbal products with medicines, with details on commonly used herbs such as ginseng, turmeric, ginkgo, St John's wort and garlic (CBG-MEB, Kruidencheck). A checker is a good way to prepare questions; it can't see your exact formulation or your health history, so it doesn't replace the conversation.

Why "just take them apart" isn't a safety plan

"Just take them two hours apart" is a popular fix, and it can help in cases where the issue is how a medicine is absorbed. It is unlikely to help with an enzyme interaction like St John's wort or an effect on serotonin, because those are not about two pills meeting in the stomach: Apotheek.nl says the pill-related effect is still present for four weeks after stopping St John's wort. Stopping matters too: the EMA warned in 2000 that, because these interactions come from enzyme induction, stopping St John's wort may raise blood levels of some medicines to the point of toxicity (it named ciclosporin, digoxin, theophylline and warfarin; EMA public statement). Ask your pharmacist whether timing even applies to your combination, and don't change or stop a prescribed medicine on the basis of an article or a seller's advice.

When you combine several ingredients

Blends and stacks add a different problem. Evidence on each ingredient separately may say nothing about the combination, and with several ingredients in play it is harder to tell what caused a reaction. Our guide to reading supplement labels shows how to find the exact amounts and ingredients to bring along. Ginkgo, a familiar botanical, is a useful reminder that it needs the same checks: NCCIH states that there is no conclusive evidence ginkgo helps any health condition, and that it may increase bleeding risk for people on anticoagulants (NCCIH, Ginkgo).

Ask again when things change, and know when to get help fast

Revisit the question when a medicine, product or amount changes, before surgery or other procedures, and during pregnancy or breastfeeding. Tell the treating team about supplements, and follow their instructions rather than inventing your own stopping or monitoring schedule.

If you suspect a reaction, contact your GP, pharmacist or out-of-hours GP service promptly. NCCIH lists agitation, diarrhoea, rapid heartbeat, raised blood pressure and hallucinations among the signs of the serotonin reaction described above. For severe or rapidly worsening symptoms, such as confusion, a very fast heartbeat, high fever, seizures or collapse, call 112 rather than trying to work it out online.

Questions people ask

Can a multivitamin interact with a medicine? Its ingredients can matter. Take the exact label to your pharmacist rather than relying on the category name.

Does no warning on the pack mean no interaction? No. A missing warning is not a compatibility check, and in the Netherlands herbal supplements need no approval before sale (CBG-MEB). Ask about the specific ingredients and medicines.

Should I stop everything before surgery? Ask the team arranging the procedure. Give them your full list and follow their instructions, as advice depends on the product and the procedure.

Where to read next

Two more places to carry on reading:

• Kanna: meet the plant behind the extracts, for a smartshop plant that comes with its own medication cautions.
• Sleep routines: the useful basics, for everyday sleep habits, where they help and when to ask for help.
• Nootropics: what belongs in the category?, for where the word came from, what healthy-adult research shows and the safety basics.

Written by Smart Supplements with AI assistance from the sources linked in the text. It has not been reviewed by a clinician.

Last updated: 12 October 2026

This article is for informational purposes only and is not intended as medical advice. Always consult a healthcare professional before starting any new supplement, especially if you take prescription medication.