My room mate has medical issues, and when he comes home from the doctors he sometimes slams me into walls and always screams at me what should I do ?

Is this a trick question? Get rid of the roommate.

I'm not sure how we are on a shared lease

Calling the police comes to mind.

Please clear up the mystery of what 'medical issues' cause a person to slam a roommate into the wall upon returning from a doctor visit?

I'm not sure how we are on a shared lease

That does make it more complicated. Unless the landlord will allow you out of the lease, you are bound by the lease. What state or country are you in?

Or, you could perhaps get a domestic violence restraining order against the room-mate requiring him to go away. That would require you to pay the full rent, however, or find another room-mate.