A patient with infectious mononucleosis developed a generalized maculopapular eruption after taking ampicillin trihydrate by mouth for eight days. This type of eruption occurs in most patients with infectious mononucleosis who are treated with ampicillin. Thus if a patient suspected of having infectious mononucleosis needs antibiotic therapy, ampicillin should be avoided.
Coskey RJ, Paul L. Ampicillin Sensitivity in Infectious Mononucleosis. Arch Dermatol. 1969;100(6):717-719. doi:10.1001/archderm.1969.01610300067010