Phenytoin is a relatively insoluble weak acid, usually administered as the sodium salt. Bioavailability is dependent upon particle size and problems of generic inequivalence have therefore arisen, particularly in Scandinavia. The drug has a moderately large volume of distribution and is approximately 90% bound to plasma proteins. Clinically important displacement can be caused by bilirubin and several drugs, particularly sodium valproate, which is often combined with phenytoin. Displacement will lower the total serum concentration but will little affect the free drug concentration.
The metabolism of phenytoin to the major metabolite, 5-(p-hydroxyphenyl)-5-(phenylhydantoin, is saturable, giving rise to a non linear dose-serum concentration relationship. Therefore, the dose range compatible with a therapeutic serum concentration is narrow within subjects, and monitoring serum concentrations is of particular value in dosage tailoring. In renal failure, the binding of phenytoin to plasma proteins is reduced and therefore a lower range of serum drug concentrations is compatible with therapeutic control. In liver disease, binding may also be impaired but delayed metabolism may occur in addition. During pregnancy the serum concentration may fall progressively as pregnancy advances, probably due to an increased rate of metabolism. Phenytoin readily crosses the placenta, and is metabolised rapidly by the neonate exposed in utero.