How Interactions Work
How medicine and supplement interactions work: decreased or increased effects, documented vs reported vs preliminary vs theoretical, narrow-range medicines, over-the-counter drugs, and why we have no interaction checker.
Content last updated . This page has not been medically reviewed.
What an interaction is
Certain dietary supplements can change absorption, metabolism or excretion of a medicine, so you may get too much or too little of it. [1] NIH describes two directions: a supplement can decrease a medicine's effect (for example St. John's wort speeds up the processes that turn drugs into inactive substances) or increase its effects and side effects. [2][3]How firm is the evidence? Our four labels
We label each interaction by what the source says, not by how worrying it sounds.| Label | Meaning | Example |
|---|---|---|
| Documented | The source states the interaction has been clearly shown | St. John's wort and many medicines [4] |
| Reported | Case reports or reports of an interaction | Licorice and corticosteroids [5] |
| Preliminary | Early or small study, or limited evidence | Ashwagandha and benzodiazepines; goldenseal and metformin in healthy adults [3][2] |
| Theoretical | A mechanism is plausible but no human data in our sources | We use this label only when a source says so |
See Interactions by drug class.
When interactions matter most
NIH singles out two situations: taking a medicine with a narrow therapeutic range, and going to have surgery. A little too much or too little of such a medicine can cause serious or life-threatening reactions. Examples: carbamazepine, cyclosporine, digoxin, phenytoin and warfarin. [6] See Supplements before surgery.Over-the-counter medicines too
Black cohosh and St. John's wort may interfere with fexofenadine (Allegra); goldenseal, St. John's wort, kratom and cannabidiol (CBD) may interact with many medications, including those available without a prescription. [7]Cancer and HIV treatment
There is growing concern that dietary and herbal supplements may interfere with the effectiveness of cancer and HIV treatments; discuss all supplements with your provider. [8]Why there is no interaction checker here
We do not offer an interaction checker because we have not found one we could validate, and an incomplete checker can wrongly reassure. Ask a pharmacist, and see MedlinePlus's Herbs and Supplements pages. [9]Frequently asked questions
Can a supplement make my medicine stop working?
Yes, some can. St. John's wort is known to decrease the effects of several medicines. [2]
References
- U.S. Food and Drug Administration. Mixing Medications and Dietary Supplements Can Endanger Your Health. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). Know the Science: Some Supplements May Decrease the Effects of Medications. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). Know the Science: Some Supplements May Increase the Effects and Side Effects of Medications. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). St. John's Wort: Usefulness and Safety. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). Licorice Root: Usefulness and Safety. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). Know the Science: Some Medication-Supplement Interactions Can Be Serious. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). Know the Science: Interactions With Over-the-Counter Medications. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NIH). Know the Science: Talk With Your Health Care Providers. Read 2026-10-22.
- MedlinePlus (National Library of Medicine). Herbs and Supplements. Page opened 2026-10-22; individual entries were not reviewed for this hub.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.