Drug interactions !

How one medicine, food, supplement or illness can change the effect of another medicine, and how to spot the dangerous combinations.

Updated 2026-11-01

A drug interaction happens when another substance or condition changes a drug's level or effect. Pharmacokinetic interactions change how much drug reaches its target (absorption, distribution, metabolism, excretion); pharmacodynamic interactions add to or oppose the effect at the target. Most serious interactions involve a narrow-index drug combined with an enzyme or transporter inhibitor or inducer, or two drugs with the same toxicity.

Pharmacokinetic interactions

Absorption: chelation (tetracyclines or fluoroquinolones with calcium, iron, antacids), changes in gastric pH
Metabolism: CYP inhibitors (clarithromycin, azole antifungals, ritonavir) and inducers (rifampicin, carbamazepine, phenytoin, St John's wort)
Transport: P-gp inhibitors raise digoxin; OATP1B1 inhibitors raise statins
Excretion: probenecid slows renal secretion of anionic drugs; cimetidine raises metformin

Pharmacodynamic interactions

Bleeding: warfarin or other anticoagulants with antiplatelets or NSAIDs
Sedation and breathing: opioids with benzodiazepines, alcohol or other depressants
Serotonin syndrome: several serotonergic drugs together
Hyperkalaemia: ACE inhibitors or ARBs with potassium-sparing diuretics or potassium supplements
QT prolongation: several QT-prolonging drugs, especially with low potassium or magnesium

Classic examples

Clopidogrel is activated by CYP2C19, and its label advises avoiding omeprazole and esomeprazole because they reduce its antiplatelet effect. Warfarin's INR rises with CYP2C9 or CYP3A inhibitors and falls with inducers. Simvastatin levels rise sharply with strong CYP3A inhibitors and grapefruit juice, raising the risk of muscle damage. Digoxin levels rise with amiodarone or verapamil.

How interactions are checked

Take a full list: prescription and over-the-counter drugs, herbal products, supplements, alcohol, diet
Read the label's drug-interaction section; labels change as new information appears
Use an interaction checker and ask the pharmacist
When a strong inhibitor or inducer must be started or stopped, plan extra monitoring of the narrow-index drug

Test yourself

Think of the answer, then tap the question to see it.

What is the difference between a pharmacokinetic and a pharmacodynamic interaction?

Pharmacokinetic interactions change the amount of drug that reaches the target; pharmacodynamic interactions change the effect at the target without changing levels.

Why is grapefruit juice a problem for some statins?

It blocks intestinal CYP3A4, so more simvastatin or atorvastatin is absorbed intact and blood levels rise, raising the risk of muscle toxicity.

What is an example of a drug–disease interaction?

A nasal decongestant can raise blood pressure in a person with hypertension.

Why is combining warfarin with an NSAID risky even without a change in INR?

It is a pharmacodynamic interaction: NSAIDs impair platelets and can injure the stomach lining, adding to the bleeding risk of the anticoagulant.

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Example drugs

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Drug Interactions: What You Should Know (U.S. Food and Drug Administration, 2026); For Healthcare Professionals | FDA’s Examples of Drugs that Interact with CYP Enzymes and Transporter Systems (U.S. Food and Drug Administration, 2026); Grapefruit Juice and Some Drugs Don't Mix (U.S. Food and Drug Administration, 2026); DailyMed - CLOPIDOGREL BISULFATE tablet (DailyMed (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.