Risk of rash with some medications. Amoxicillin and other antibiotics, including those made from penicillin, aren't recommended for people with mononucleosis. In fact, some people with mononucleosis who take one of these drugs may develop a rash. The rash doesn't necessarily mean that they're allergic to the antibiotic, however. If needed, other antibiotics that are less likely to cause a rash are available to treat infections that may go along with mononucleosis.