I had an allergic reaction to epoxy resin for crafts, how long before a cortisone shot clears up a severe rash on my hands and wrists?

Contact dermatitis: It really depends on several factors. Did you get all of the material off your skin? How long was it in contact with your skin? Is this the first or 2nd time you have had this reaction? How much "cortisone" was in the injection? Most reactions clear in a week. Sometimes a 2nd injection may be necessary. In that case, figure two weeks. But is should get better each day.

Few days.: It depends on the medication used, but the most common, triamcinolone, takes 2 to 4 days to start working. In the meantime you can soak your hands in a basin of warm water a couple of times a day. I hope your doctor also gave you a topical anti-inflammatory, and something for the itching.