Is Excedrin pm good for arthritis in the arm?

It's a pain reliever. It can help some depending on what type of arthritis one has. It's not an anti-inflammatory but an analgesic with Acetaminophen ( which is also in tylenol) and Diphenhydramine (which is in benadryl) and is an antihistamine and can make people drowsy and is used to help sleep (the pm component).For osteoarthritis Acetaminophen can help some people.
Probably not. Excedrin pm is a nigttime medication for headaches - there is probably no reason for the caffeine (for headaches) or the pm component. A number of over the counter meds such as tylenol (acetaminophen) arthritis are a better first choice if you do not have other issues - discuss with your pharmacist.