epilepsy
"After 41+years on various meds many included this one.  Unfortunately, it must be monitored closely and depending on factors like body weight, other meds taken, caffeine intake, and more, it can be a difficult balancing act.  Dilantin toxicity has happened and is a living hell for all.  The med can be good to stop nocturnal tonic clonics if you are having those, which are life-threatening.  Do your research, make sure dilantin levels are taken periodically at trough and consider more than just the 20 max if you take a does afterward.  Make sure you have a liver profile taken periodically.  A doc may have to be reminded but it's best to have one who knows meds and does bloodwork regularly. Find a good epileptologist you get along with."