<p>May reduce the efficacy of&nbsp;oral contraceptives. May alter INR while on warfarin and phenindione. May reduce the efficacy of oral typhoid vaccines. May reduce the excretion of methotrexate. Reduced excretion with probenecid and sulfinpyrazone, resulting to increased risk of toxicity. Allopurinol increases ampicillin-induced skin reactions. Reduced absorption with chloroquine. Bacteriostatic antibacterials (e.g. erythromycin, chloramphenicol, tetracycline) may interfere with the bactericidal action of ampicillin.</p>