Often doctors find that medicines are useful for purposes other than the ones approved by the Food and Drug Administration (FDA). It is legal to prescribe medicines for these "off-label uses."

Phenytoin has two common off-label uses:

1. It may be helpful in treating a kind of face pain called trigeminal neuralgia. The seizure medicine carbamazepine (Tegretol or Carbatrol) is usually more effective, however.
2. Doctors sometimes use phenytoin to treat patients with certain psychiatric disorders, such as the mania of bipolar disorder. Few studies have shown a definite benefit, however. Another reason for caution in this kind of use is that phenytoin interacts with many of the medicines generally used to treat psychiatric disorders.