How long do the side effects of prednisone last? It caused a bad rash (check the possible side effects if u don't believe me) and it won't go away.

Dose dependent. It is usually a few days. Everybody reacts differently. Sorry. If you feel your breathing worsening or heart fluttering you will want to be checked. Some concern is why were you started on the steroid and is this a reaction or symptom of the problem that was coming. Just be safe.
Can’t tell from info. With the information provided, it’s hard to tell. Contact your doctor - it’s possible you experienced a mild allergic reaction to the drug. If that’s the case, it’s important to have that documented and to determine whether you can take any similar drugs. Either way, s/he will be able to further assess your rash, including providing you a timeframe for it to go away with appropriate treatment.