Absorption
Topamax is absorbed well and peak plasma levels are reached in 6 to 8 hours.

Distribution and metabolism
Topamax metabolism depends on the hepatic P450 system.

Nearly 70 to 80% of Topamax is excreted unchanged in the urine, so the clearance of Topamax is reduced by renal insufficiency. The half-life of Topamax is approximately 21 hours with normal renal function.

Steady state
Steady state is reached after 4 to 5 days in patients with normal renal function. 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.