Could a cortisone shot in the left shoulder cause loss of arm use. it's been 5 months and still no change. ?

It would be...: Extremely rare. You need to find the real cause. An orthopedist would be a good start. You did not mention what is causing arm disuse! pain! numbness! etc.

Family practice : A single cortisone injection probably didn't cause increased discomfort and disuse of your arm if done properly. It may have been injected injuring a nerve. Have it checked out.