[Infectious mononucleosis and ampicillin].

One to 10% of the cases have cutaneous reactions caused by ampicillin. When this drug is associated to MI the percentage goes up to 90% of the cases. The physiopathogenic mechanism of the generalized exanthema in this association is unknown. The development of toxicity is postulated because these patients have no previous history of allergy to penicillin either before of after the cutaneous reaction. By a still obscure mechanism, MI predisposes or alters the patient sensibility to the drug and the cutaneous reaction appears. This picture cannot be attributed to MI because the exanthema that present is mild. Neither is this picture of allergy due to ampicillin, without relationship with the MI, because the reaction is not coincident with other descriptions of reactions by this drug. The clinic evidence, observed in patients with MI, and ampicillin, is of a strong relation between two factors and the cutaneous reaction. In our patient the eruption affected principally the sites exposed to the sun, that makes us think in a phototoxicity reaction. Despite this fact the tests done were negative. We agree with other authors that the administration of ampicillin in patients with ill-defined pharyngeal symptoms is contraindicated until infections mononucleosis is ruled out.