Phenytoin, which is used primarily as an anticonvulsant agent, has a relatively low therapeutic index, and monitoring of plasma phenytoin concentration is often used to help guide therapy. It has properties which predispose it to an involvement in pharmacokinetic interactions, a large number of which have been reported. These properties include: low aqueous solubility and slow rate of gastrointestinal absorption; a relatively high degree of plasma protein binding; a clearance that is non-linear due to saturable oxidative biotransformation; and the ability to induce hepatic microsomal enzymes. Because of its narrow therapeutic range, drug interactions leading to alterations in plasma phenytoin concentration may be clinically important. Such interactions have often been reported initially as either cases of phenytoin intoxication or of decreased effectiveness. Drugs may modify the pharmacokinetics of phenytoin by altering its absorption, plasma protein binding, or hepatic biotransformation; alterations in the absorption and/or biotransformation may lead to changes in both the unbound plasma phenytoin concentration and, as a result, the clinical effect. Preparations which may decrease the gastrointestinal absorption of phenytoin include nutritional formulae and charcoal. There are many reports of drugs which may increase (e.g. folic acid, dexamethasone and rifampicin) or decrease (e.g. valproic acid, sulthiame, isoniazid, cimetidine, phenylbutazone, chloramphenicol and some sulphonamides) the metabolism of phenytoin. It is important to bear in mind that, as a result of its non-linear clearance, changes in phenytoin absorption and/or biotransformation will lead to more than proportionate changes in plasma drug concentration. Drugs which may displace phenytoin from plasma albumin include valproic acid, salicylic acid, phenylbutazone and some sulphonamides. Although an alteration in the unbound fraction of phenytoin in plasma would not, in itself, be expected to alter the unbound plasma phenytoin concentration, the interpretation of total plasma concentrations for therapeutic drug monitoring may be confounded. Some drugs appear to alter phenytoin pharmacokinetics via dual mechanisms (e.g. valproic acid and phenylbutazone), while for other compounds the mechanism of interaction has not been fully elucidated. Phenytoin has been reported to alter the pharmacokinetics of a large number of drugs. The majority of these interactions arise because phenytoin is a potent inducer of cytochrome P450 microsomal enzymes, and therefore may increase the clearance of drugs which are extensively metabolised; drugs affected include carbamazepine, theophylline, methadone, prednisolone, dexamethasone, metyrapone and several cardiac antiarrhythmic agents. With all of these, the resultant decrease in plasma concentrations may be clinically important. Current evidence suggests that the various cytochrome P450 isozymes may be affected differentially and, therefore, that the metabolic clearance of some drugs may be more sensitive to the enzyme-inducing effects of phenytoin. In addition, phenytoin also appears to increase the clearance of some drugs which are eliminated predominantly by conjugation reactions [e.g. paracetamol (acetaminophen) and oxazepam]. Interestingly, phenytoin has been reported to decrease the metabolic clearance of a commonly coadministered anticonvulsant, phenobarbital. For a few compounds only [e.g. furosemide (frusemide) and thyroxine], it has been suggested that phenytoin decreases gastrointestinal absorption. Although the binding of drugs to plasma albumin is largely unaffected by this agent, plasma levels of α1-acid glycoprotein and sex hormone binding globulin may increase during phenytoin treatment, thereby altering the binding of some compounds.