Drug interactions are an avoidable cause of patient harm. Harm may occur due to either increased drug effect causing toxicity or decreased drug effect leading to therapeutic failure. Drug interactions should be considered both in the differential diagnosis of symptoms (for interactions that have already occurred) and when prescription changes are made (for potential interactions). Software checkers for drug interactions are widely available, but have limited clinical utility. Patient harm from drug interactions can be reduced by: using a personal formulary - using few drugs and knowing them well recognising drugs that are major perpetrators of interactions recognising narrow therapeutic index drugs as vulnerable to interactions applying clinical pharmacology principles.