I'm taking dexamethasone and now am experiencing trouble breathing and severe pain and weakness in both legs and arms is it related to the drug?

Probably: Steroids are a double edged sword, meaning they can fix some problems, and cause others. Sometimes they imrove breathing, but they can also cause different breathing problems, sometimes alleviate pain, sometimes cause it. Some of the side effects can be dangerous, so i suggest you speak with your doctor about it.

Side effects: Your drug is a steroid, and side effects are not unusual. Problems with muscle weakness usually occurs with chronic steroid usage and is dosage dependent. Fluid and chemical changes may be a problem. Contact your physician regarding your problems, as there are possible lung complications and these need to be evaluated.