contraindication

A contraindication is a specific reason not to use a particular drug or treatment in a particular patient — a 'red light' in the prescribing instructions. It flags a situation where the expected harm clearly outweighs the benefit, so the drug should be avoided.

Contraindications come in degrees. An absolute contraindication means the drug must not be used under any circumstances, because the risk is unacceptable — giving a known anaphylaxis-inducing penicillin to someone with a true penicillin allergy, for instance. A relative contraindication means caution is needed and the drug should be used only if the benefit truly justifies the risk, often with extra monitoring or a lower dose.

They arise from many sources: a coexisting disease (an NSAID in active peptic ulcer disease), an organ that cannot handle the drug (a renally cleared drug in kidney failure), pregnancy and breastfeeding, age extremes, a documented allergy, or a dangerous drug interaction. Each is essentially a statement that this patient sits in the wrong part of the risk-benefit balance.

Contraindications are the practical bridge between a drug's known toxicity and a real patient. They are listed in the product label precisely so that the dangers learned in trials and post-marketing surveillance translate into safer everyday prescribing — but they require a clinician to know the patient, not just the drug.