Avoiding drug interactions

(1) Drug interactions potentiate or oppose the therapeutic or adverse effects of one or more co-administered drugs. (2) There are two mechanisms: pharmacodynamic interactions occur between drugs that have similar actions or adverse effects. The shared effect can potentiate or antagonise the actions of a physiological system such as clotting, or the parasympathetic nervous system. Pharmacokinetic interactions alter the metabolism of the relevant drug(s). (3) The clinical consequences of drug interactions are very variable. Most interactions with serious consequences involve either high risk drugs or high risk patients (such as older people). (4) The risk of interactions increases with the number of drugs taken, and with advancing age. (5) Some patients are at an increased risk of drug interactions because they have pathology that alters drug metabolism, such as renal failure, liver failure or malnutrition. Other patients are at risk because of the type and number of drugs they need (this is particularly true for patients with heart disease, HIV infection, epilepsy, tuberculosis and migraine). (6) In practice, patients must be questioned thoroughly about all the drugs they are taking, whether or not they have been prescribed.