Seizures
"I have tonic-clonic seizures due to a malignant tumor in my frontal lobe.  I was prescribed phenytoin, which controlled the seizures but my doctors could not get my blood levels stable so I was constantly oscillating from sub-therapeutic to toxic.  After 6 months of trial and error with no success, I was switched to Keppra.  Keppra failed to control the seizures and I became hyperactive, easily agitated, and began having suicidal thoughts. My doctor made me switch back to phenytoin. The FDA recently required the manufacturer of Keppra to change the warning label to include these as known adverse reactions in some patients."