seizures
"Brain rot. I have been on dilantin for over 40 years. Have just discovered dilantin is cause to numbness in legs, feet, balance and walking control. How so?" It's called cerebellum atrophy.  Thats where your cerebellum shrinks (my case about 40%) 
It will get worse if I continue to take it, putting me in a wheel chair. Only one medicine does that to you- Dilantin. DO NOT TAKE IT. If you are on it, get off it (as I am about to do) It was also considered at one time the most toxic medicine out there. Look it up in PDR. Did it stop seizures? yes. Do I want to be a cripple and eventually have to be fed. No. Get off it."