COMMONLY USED. Ampicillin is a betalactam antibiotic that is part of the amino-penicillin family and is roughly EQUIVALENT to its successor, AMOXICILLIN in terms of spectrum and level of activity. It can sometimes result in reactions that range in severity from a rash (in the case of PATIENTS that may unwittingly have mononucleosis) to potentially lethal allergic reactions such as anaphylaxis. However, as with other penicillin drugs, it is relatively non-toxic and adverse effects of a serious nature are encountered only rarely.