Absorption
Phenytek is an extended-release form of capsules intended to be taken once a day. It is reported to be bioequivalent to other phenytoin sodium capsules, such as Dilantin. Each Phenytek capsule actually contains two or three 100-mg erodible-matrix tablets that gradually release the medication over a 24-hour period. Peak plasma levels are reached 4 to 12 hours after a capsule is taken. Food does not affect the amount of the medication that is absorbed.

Distribution and metabolism
Phenytoin is 69% to 96% bound to plasma proteins (average 90%), and metabolized in the liver.

At low concentrations, elimination is proportional to the concentration. But as the concentration rises to therapeutic levels, the enzyme system in the liver can become saturated and elimination occurs at a constant rate despite the dose (zero order kinetics). The effect is that a small increase in the dose sometimes causes a large change in the level of free phenytoin in the blood.

Most phenytoin is excreted in the urine in the form of inactive metabolites. The half-life is about 22 hours, with a range of 7 to 42 hours. The half-life is shorter in children and longer in the elderly.

Because phenytoin is metabolized in the liver, people with liver disease should be treated with caution. They may have to be started at a lower dose and have their dosage increased more slowly.

The level of phenytoin in the blood can vary widely between different patients who have been prescribed the same dosage. Possible reasons for these differences include:

• failure to follow the dosing schedule
• interaction with other medications
• liver disease
• enzyme deficiencies
• differences in the way each person's body metabolizes phenytoin.

Steady state
Steady state is reached after 5 to 8 days (5 to 7 half lives) of taking a stable dose of phenytoin. The dose that a patient takes should not be increased until steady state has been reached (or some time later), so that the effects of the previous dosage can be assessed.