Phenytoin is a drug used to control epileptic seizures. Several conditions can affect the blood levels and efficacy of physician-prescribed phenytoin: variations in intestinal absorption, liver diseases, protein binding rates, combinations with other drugs, etc. The phenytoin test measures blood levels of the drug at least 10 hours after the last dose (unless otherwise indicated by the physician), and results are expressed in micromoles of phenytoin per litre of blood (µmol/L).
Most people respond adequately and with minimal side effects to a phenytoin level between 40 and 80 µmol/L (measured at least 10 hours after the last dose). In some people, this therapeutic window will not adequately control seizures, while other people will experience excessive side effects even if the level remains within the therapeutic window. Side effects vary widely and include loss of balance and falls, uncontrolled eye movement from right to left (nystagmus), confusion, incoherent speech, tremors, low blood pressure, etc.