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How to Check an Interaction Safely: Pharmacists and Official Resources

Why this site has no interaction checker, what a pharmacist or prescriber can check for you, and which official pages (NCCIH, MedlinePlus, NHS, FDA) are worth reading.

Content last updated . This page has not been medically reviewed.

Quick answer
The safest way to check a supplement against your medicines is to ask a pharmacist or your prescriber, with a written list of everything you take. NCCIH names health care providers and pharmacists first among its resources on supplement-medicine interactions. [1] You do not need to stop or change any prescribed medicine to ask; if a change is needed, the prescriber decides it.

If you feel very unwell after combining products, call 911 for collapse, a seizure or trouble breathing, 988 (US) for a mental health crisis, or Poison Help at 1-800-222-1222 (US) for possible poisoning.

Why this site has no interaction checker

We do not offer a checker. Three reasons come from what we read:
  • Plants and products are missing from some tools. A 2026 study of 658 patients in Vietnam compared four commonly used drug interaction databases and found only 0.7% of flagged interactions were consistently documented in all four, and many commonly used local plants were absent from all of them. [2] That is one study in one setting, so it does not rate any single tool; it does show that "nothing flagged" can depend on the tool.
  • Many supplements were never tested with medicines. The NHS says, on its lithium and digoxin pages, that herbal remedies and supplements are generally not tested for the effect they have on other medicines. [3] [4]
  • Sources change. MedlinePlus notes that, as of July 29, 2025, the herbs and supplements information from the Natural Medicines Comprehensive Database is unavailable, and it now points to NCCIH for many entries. [5]

What to bring

  • A written list of every prescription and over-the-counter medicine, vitamin and supplement, with how often you take each and how much. [6]
  • The bottles themselves, so ingredients can be checked. [6]
  • Anything you use sometimes, such as sleep aids, pain relievers, teas, oils and capsules. FDA says to bring the list every time you visit. [7]

If you are treated for cancer or HIV, NCCIH says to discuss every supplement, because of growing concern that supplements may interfere with those treatments. [6] A printable medicine and supplement list is available.

Official pages worth reading

ResourceWhat it offersRead this session?
NCCIH herb and supplement pagesSafety sections with interaction statements for single herbs, such as valerian or ginkgo. [8]Yes
MedlinePlus drug pagesEach medicine page has a section listing herbal and nonprescription products that may interact, for example warfarin or digoxin. [9] [10] MedlinePlus is run by the National Library of Medicine. [11]Yes
NHS medicine pagesPages named "taking [medicine] with other medicines and herbal supplements", for example lithium. [3]Yes
FDA consumer updatesPlain-language warnings such as mixing medicines and supplements, and grapefruit. [7] [12]Yes
NIAAA alcohol and medicines listMedicines grouped by condition with possible reactions to alcohol. [13]Yes
LiverToxA liver injury reference that NCCIH cites for valerian. [14]Cited only; page could not be opened
LactMedNot described hereCould not be opened

How to use LiverTox, LactMed and the others is explained in how to use NCCIH, MedlinePlus, LiverTox and LactMed.

What a result means

  • A listed interaction is a reason to ask, not an instruction to stop anything. See how interactions work for documented, reported, preliminary and theoretical.
  • No interaction found is not proof of safety. The NHS says there is not enough information to say other herbal remedies and supplements are safe with lithium or digoxin. [3] [4]
  • Your own product may differ. NCCIH notes supplements from stores or online can differ from products tested in studies. [15] See quality testing and certifications.

Limits of what we found

Proper evidence not found in the pages we read for a step-by-step method that pharmacists use to check herb-medicine interactions, or for how accurate any single online checker is. We searched NCCIH, MedlinePlus, FDA, NHS and PubMed. For the medicine groups, see interactions by drug class.

Before surgery and in other situations

Dietary supplements may interact with medicines or pose risks for people with medical problems or before surgery. [15] Tell the surgical and anesthesia team about every product. See supplements before surgery.

Frequently asked questions

Is there a quick way to check one supplement?

Start with the NCCIH page for that herb and the MedlinePlus page for your medicine, then ask a pharmacist. [1]

Are free online interaction checkers reliable?

Proper evidence not found for all checkers. One 2026 study found poor agreement across four databases in one setting. [2]

Who is the best person to ask?

A pharmacist or your prescriber. NCCIH lists them first. [1]

What if I did not tell my doctor I use herbs?

You are not alone: in the 2026 Vietnam study, 48.6% of participants used herbal medicines and 96% of those did not tell their health care providers. [2] Telling them now helps; see talking with a health professional.

Does this site replace a pharmacist?

No. This page is information only.

References
  1. NCCIH, Know the Science: How Medications and Supplements Can Interact, Additional Resources (NCCIH). Read 2026-10-22.
  2. PubMed, Herbal medicine use disclosure, database-flagged potential herb-drug interactions, and inter-database concordance among patients with non-communicable diseases in Vietnam (PLoS One, 2026). Read 2026-10-22.
  3. NHS, Taking lithium with other medicines and herbal supplements (NHS, page last reviewed 9 August 2023). Read 2026-10-22.
  4. NHS, Taking digoxin with other medicines and herbal supplements (NHS, page last reviewed 22 March 2023). Read 2026-10-22.
  5. MedlinePlus, Herbs and Supplements (MedlinePlus). Read 2026-10-22.
  6. NCCIH, Know the Science: How Medications and Supplements Can Interact, Talk With Your Health Care Providers (NCCIH). Read 2026-10-22.
  7. FDA, Mixing Medications and Dietary Supplements Can Endanger Your Health (FDA Consumer Update, content current 2 June 2022). Read 2026-10-22.
  8. NCCIH, Ginkgo: Usefulness and Safety (NCCIH, updated February 2025). Read 2026-10-22.
  9. MedlinePlus, Warfarin: MedlinePlus Drug Information (last revised 15 March 2026). Read 2026-10-22.
  10. MedlinePlus, Digoxin: MedlinePlus Drug Information (last revised 15 June 2017). Read 2026-10-22.
  11. MedlinePlus, About MedlinePlus (National Library of Medicine). Read 2026-10-22.
  12. FDA, Grapefruit Juice and Some Drugs Don’t Mix (FDA Consumer Update, content current 1 July 2021). Read 2026-10-22.
  13. NIAAA, Harmful Interactions: Mixing Alcohol With Medicines (NIAAA, published 2003, revised 2014). Read 2026-10-22.
  14. NCCIH, Valerian: Usefulness and Safety (NCCIH, updated May 2025). Read 2026-10-22.
  15. NCCIH, Using Dietary Supplements Wisely (NCCIH, updated January 2019). Read 2026-10-22.
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.