Contraindications: Allergy to penicillin or cephalosporins Anaphylaxis and death are possible outcomes. The risk is greatest in those who report previous allergic reactions to penicillin and less in those previously reacting to cephalosporins. Mononucleosis Rashes occur in a high percentage of patients with mononucleosis who are given amoxicillin. Penicillinase-producing Staphylococcus strains These organism are resistant to amoxicillin.