B

Diarrhoea is the most frequent adverse effect of oral ampicillin because the drug disturbs normal intestinal flora; prospective studies report diarrhoea in 15–20 % of recipients, far higher than rates of anaphylaxis, oral candidiasis, or renal toxicity [Lusk et al., 1977, PMID 850084; Tedesco, 1975, PMID 1130357].