Many oral antibiotics cause rashes as side effects. Amoxicillin, however, is usually the worst culprit. It is part of the penicillin family. This is the most common allergy producing medication. 10 percent of people are reported to be allergic to penicillin. That is usually overreported, however, as many patients mistakenly think they are having an allergic reaction to penicillin. In fact, it is only a common side effect, or reaction that happens to many people. afer using penicillin.