Phenytoin (diphenylhydantoin) has been used extensively for seizure control in patients having both grand mal epilepsy (major motor), cortical focal seizures, and temporal lobe epilepsy. Serum level monitoring of the drug is essential in order to achieve maximal seizure control while maintaining minimal blood levels. Because of individual variation in absorption and metabolism, optimum levels may vary.
Expected values
The therapeutic range of phenytoin is correlated with seizure control as well as the absence of toxic effects, and is generally accepted to be between 10 and 20 μg/mL (39.6 and 79.2 μmol/L). Because of individual variation in absorption and metabolism of the drug, optimum levels may vary and reach higher than 20 μg/mL (79.2 μmol/L) or they may fall below 10 μg/mL (39.6 μmol/L). Toxic signs are seldom seen below 15 μg/mL (59.4 μmol/L), while nystagmus often appears when serum levels rise above 20 μg/mL (79.2 μmol/L). Ataxia is observed most often when serum levels reach 25 to 30 μg/mL (99 to 119 μmol/L) and somnolence and dysarthria above 40 μg/mL (158 μmol/L). At high doses, phenytoin can even cause an increase in the frequency of seizures.
| Micromole per liter | µmol/L = mcmol/L = umol/L = µM/L = mcM/L = uM/L = micromol/L |
| Milligram per liter | mg/L = millig/L = milligram/L = mg/liter |
| Milligram per deciliter | mg/dL = millig/dL = milligram/dL |
| Milligram per 100 milliliters | mg/100mL = millig/100mL = milligram/100mL |
| Milligram percent | mg% = millig% = milligram% |
| Microgram per milliliter | µg/mL = mcg/mL = ug/mL = microg/mL = microgram/mL |