Normally, your doctor will tell you to start by taking 60 milligrams (mg) each day. After a while the doctor may increase the dosage to get better control of your seizures. However, at doses of over 90-120 mg per day, many adults will develop troublesome tiredness and other side effects, so you may want to ask about using a different medicine if these effects seem to be outweighing the benefit of the phenobarbital.

Because of individual differences between people and differences in the type and severity of epilepsy, however, there is no single "best" dosage and no "therapeutic range" of blood levels that applies to everyone. Some patients are seizure-free with low levels while others who have a higher level of medication in their blood continue to have seizures. Patients who have only tonic-clonic seizures usually achieve seizure control with lower levels of phenobarbital than those who have simple or complex partial seizures.

An uncommon but serious side effect of phenobarbital is a possibility of becoming addicted to the medicine. Therefore, it is important to take only the dosage that your doctor prescribes.

The doctor will calculate the dosage for a child based on the child's weight. The usual dose for children is 1 to 6 mg per kilogram (2.2 pounds) of body weight per day.

Reviewed February 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.