How Interactions Work

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What an interaction is

An interaction means one product changes how another works. The FDA explains that certain dietary supplements can change how a medicine is absorbed, broken down (metabolized) or removed (excreted), so a person may get too much or too little of the medicine. [1] The NIH's National Center for Complementary and Integrative Health (NCCIH) describes three possible results: the medicine's effect goes up, goes down, or the combination acts in a harmful way. [2]

How interactions happen, in plain terms

• A supplement speeds up the body's clearing of a medicine. NCCIH says St. John's wort decreases the effects of some drugs by speeding up the processes that change drugs into inactive substances. [3] A review of its drug interactions describes this as activation of a receptor that switches on drug-processing enzymes and a drug-moving protein. [4] Three of that review's four authors work for a maker of a St. John's wort product, which is worth knowing when reading it.
• A food or supplement slows the body's clearing of a medicine. The FDA says grapefruit juice can block an enzyme in the gut called CYP3A4, so more of some drugs enters the blood and stays longer. [5] See Grapefruit and medicine interactions.
• Two products push the same body system. The FDA says warfarin, ginkgo, aspirin and vitamin E can each thin the blood, and taking them together may raise the chance of internal bleeding or stroke. [1] MedlinePlus says serotonin syndrome most often occurs when two or more medicines that affect serotonin are taken together. [6]

How firm is the evidence? Our four labels

We label each statement by what the source says, not by how worrying it sounds. The labels are this site's own way of sorting, not an official scale.

When a source only warns, without study detail, we say "the source cautions" and name the body. Our class pages use the same labels, starting from Interactions by drug class.

When interactions matter most

NCCIH singles out two situations: taking a medicine with a "narrow therapeutic range," and going to have surgery. For a narrow-range medicine, even a little too much or too little can cause serious reactions. NCCIH's examples are carbamazepine, cyclosporine, digoxin, phenytoin and warfarin. [11] If you take one of these, ask the prescriber or pharmacist before adding anything new. See Narrow-range medicines and supplements and Supplements before surgery.

Over-the-counter medicines count too

NCCIH notes that black cohosh and St. John's wort supplements may interfere with the effectiveness of fexofenadine, an allergy medicine sold without a prescription. It adds that goldenseal, St. John's wort, kratom and cannabidiol (CBD) may interact with many medications, including those sold without a prescription. [12]

Cancer and HIV treatment

NCCIH says there is growing concern that dietary and herbal supplements may interfere with the effectiveness of cancer and HIV treatments, and that anyone treated for either should discuss all supplements with their provider. [13]

No news is not good news

A lack of reports does not show that a combination is safe. The NHS digoxin page says there is not enough information to say that other herbal remedies and supplements are safe to take with digoxin, because they are generally not tested for their effect on other medicines. [14] A small trial of eight healthy volunteers found no significant change in digoxin measures when a hawthorn extract was added. [15] That is one preparation, a small group and a short study, so it cannot speak for other hawthorn products or for people who are ill. A review of nine trials of herbs with warfarin found it difficult to say whether ten herbal medicines had significant effects, because of low-quality evidence. [16]

What a reader can do

Bring a written list of everything you take (prescription, over-the-counter, supplements), or the bottles themselves, to every visit. [13] Ask a pharmacist, who can check your specific medicines. See How to check an interaction safely. Never start, stop or change a prescribed medicine on your own; ask the prescriber or pharmacist.

Why this site has no interaction checker

We do not offer an interaction checker, because an incomplete checker can falsely reassure. MedlinePlus keeps an index of herbs and supplements that links to NCCIH pages. [17] Quality of the product matters too; see Quality testing and certifications.