What a drug interaction is
A drug interaction happens when one substance changes the effect of a medicine. The other substance can be another drug, a food or drink, a herbal product, or even a disease state. Interactions can make a drug stronger, weaker, or more toxic.
Interactions also affect lab tests: some drugs change a test result without any real change in the patient, which can mislead clinical decisions.
Two main mechanisms
Pharmacokinetic interactions change how much drug reaches its target — for example by altering absorption, protein binding, metabolism, or excretion. Pharmacodynamic interactions happen at the site of action, when two drugs add to or oppose each other's effects.
- Enzyme inhibition — a CYP inhibitor slows the metabolism of another drug, so its levels rise. Examples of strong inhibitors include clarithromycin and ketoconazole. [3]
- Enzyme induction — an inducer such as rifampicin, carbamazepine, or phenytoin speeds up metabolism, so levels of the other drug fall (for example, oral contraceptives can fail). [1,3]
- Absorption — calcium, iron, and antacids bind (chelate) tetracyclines and fluoroquinolones in the gut, reducing how much is absorbed. [2]
- Additive effects — two drugs that both cause sedation, bleeding, or QT prolongation are riskier together than either alone.
Food and drink interactions
- Grapefruit juice inhibits CYP3A4 in the gut wall and can raise levels of some statins and calcium channel blockers. [1,4]
- Tyramine-rich foods such as aged cheese can trigger a hypertensive crisis in people taking MAO inhibitors. [2]
- Large changes in vitamin K intake (leafy green vegetables) affect warfarin's effect — the advice is consistency, not avoidance. [1]
- Alcohol adds to the sedative effect of many drugs and can interact with metronidazole.
High-risk combinations worth knowing
- Warfarin with NSAIDs or many antibiotics — higher bleeding risk. [1,2]
- ACE inhibitors or ARBs with potassium-sparing diuretics or potassium supplements — hyperkalaemia. [2]
- Two or more serotonergic drugs (for example an SSRI with tramadol) — serotonin syndrome. [1]
- Several QT-prolonging drugs together — risk of dangerous arrhythmias.
- Opioids with benzodiazepines or alcohol — respiratory depression. [4]
The checks that catch interactions
Safe practice layers several checks: an up-to-date medication list (including over-the-counter and herbal products), an allergy check, software interaction screening at prescribing, and an independent pharmacist review before dispensing. Each layer catches problems the others miss.
Red flags to escalate
- Unexpected bleeding or bruising in a patient on anticoagulants.
- Agitation, fever, tremor, and muscle rigidity after starting a serotonergic drug.
- Palpitations or fainting after adding a QT-prolonging medicine.
- Very slow breathing or unusual drowsiness in a patient on opioids.
References
- 1.Stockley's Drug Interactions. London: Pharmaceutical Press (current edition).
- 2.Joint Formulary Committee. British National Formulary (BNF), Appendix 1: Interactions. London: BMJ Group and Pharmaceutical Press.
- 3.U.S. Food and Drug Administration. Drug Development and Drug Interactions: Table of Substrates, Inhibitors and Inducers. Silver Spring (MD): FDA.
- 4.DailyMed: FDA-approved drug labels. Bethesda (MD): U.S. National Library of Medicine.
For education only. This is not a substitute for professional training, supervision, or local clinical guidelines.