Ampicillin may reduce the efficacy of oral contraceptives. Probenecid decreases the renal secretion of Ampicillin thus concurrent use with Ampicillin may result in increased and prolonged blood levels of Ampicillin. Concurrent administration of allopurinol during treatment with Ampicillin can increase the likelihood of allergic skin reactions.

When testing for the presence of glucose in urine during Ampicillin treatment, due to the high urinary concentrations of Ampicillin, false positive readings are common with chemical methods. Therefore, enzymatic methods should be used.