Interaction Checker

How Interaction Checker works

Most interaction checkers give you a verdict and ask you to trust it. This one shows you the sentence from the FDA label that produced the verdict, so you can judge it yourself and take it to a pharmacist. Here is the method, in full.

Sources

The primary source is the FDA prescribing label for each drug, retrieved from the openFDA drug label dataset, which mirrors the Structured Product Labeling documents published on DailyMed by the National Library of Medicine. Label text is a work of the U.S. government and is in the public domain. For each drug we choose one label: the reference (NDA or BLA) label where one exists, otherwise the most recent, most complete generic label. The chosen label, its effective date, and a DailyMed link are shown on every drug page.

Supplements, vitamins, foods, alcohol, and tobacco do not have FDA labels. For those we do two things: read the drug labels for mentions of them (labels frequently name alcohol, grapefruit, St. John's wort, antacids, and mineral supplements), and add a small set of curated rules taken from NIH Office of Dietary Supplements fact sheets, NCCIH herb pages, the FDA's consumer guidance on grapefruit, and the NIAAA's guidance on alcohol. Each rule cites the page it came from.

The matching method

  1. Sections. From each label we keep the boxed warning, contraindications, warnings and precautions (or the older warnings and precautions sections), and drug interactions. Pharmacokinetic tables are dropped. Adverse reaction lists, dosing, and clinical pharmacology are not scanned, because they rarely describe an actionable interaction.
  2. Terms. Every drug carries a set of name terms (generic name, salt form, and well-known brands) and class terms (for example "NSAID", "ACE inhibitor", "strong CYP3A4 inhibitor"). Classes come from our curated list and from the openFDA pharmacologic class fields where those are consistent across a drug's labels.
  3. Scan, both directions. For a pair A and B, each sentence of A's label is checked for B's terms, and each sentence of B's label for A's terms. Matching is whole-word, so "codeine" does not match "hydrocodone". When two drugs share a class, a class mention only counts if the sentence talks about "other" members of it, as in "other NSAIDs".
  4. Grade. A sentence in a boxed warning or contraindications section is Major. Elsewhere, wording such as contraindicated, avoid, not recommended, fatal, or serotonin syndrome is Major; monitor, dose, reduce, increase, or risk is Moderate; anything else is Minor. Sentences that say there was no clinically significant effect are graded as reporting no interaction. The pair's overall grade is the most severe sentence found.
  5. Show the work. Up to five sentences per label direction are displayed with the section they came from, the term that matched, and whether the match was by name or by class.

Coverage today

586 drugs and 30 supplements, foods, and habits are indexed, producing 42,465 pairs with documented evidence. Label data was exported from openFDA on 2026-09-03 and this site was generated on 2026-09-04. The checker can also look up any other drug with an FDA label on demand; those requests go from your browser straight to api.fda.gov.

Known limits

  • Labels are not exhaustive. A manufacturer documents what it studied or was required to warn about. A pair with nothing documented has not been shown to be safe.
  • Wording is graded by rule, not by a pharmacist. The grading rules are deliberate and consistent, but a sentence like "avoid abrupt discontinuation" will grade Major even when it is not about the other drug's danger. That is why the sentence is always shown.
  • Class matches can over-reach. A label that warns about "diuretics" applies to every diuretic in our list, including ones for which the risk is small.
  • One label per drug. Formulations differ. The label used is named on each page; an extended-release or topical product may carry different warnings.
  • Doses, timing, and your physiology are not modeled. Nothing here replaces a pharmacist's review of your actual regimen.

Who makes this

Interaction Checker is built and maintained by Elowin Labs LLC, a small software studio. It is not affiliated with the FDA, the NIH, or any pharmaceutical company, and it has no sponsors and no advertising. Corrections and questions: hello@elowinlabs.com.

Method last updated 2026-09-04.