BCG vaccine: the combination is never to be used as it presents high likelihood of serious life threatening interaction. The combination could only be used if no other alternatives are available or if the benefits truly outweigh the risks. Amoxicillin decreases the effects of BCG vaccine live by pharacodynamic antagonism. These warnings go for BCG live intravesical as well.
Tetracyclines: tetracycline class of antibiotics is strictly not to be used with amoxicillin/clavulanate especially when bactericidal action is required. Tetracyclines decrease the effects of amoxicillin by pharmacodynamic antagonism.
Methotrexate: amoxicillin increases levels of methotrexate by decreasing the renal clearance. Increased serum concentrations of methotrexate with concomitant hematologic and gastrointestinal toxicity have been seen with concurrent administration of high or low doses of methotrexate and penicillins.
Warfarin: amoxicillin increases the effects of warfarin by enhancing the anticoagulant effect of vitamin K antagonists thus increasing the risk of bleeding. INR should be monitored.
Mycophenolate: amoxicillin decreases the levels of mycophenolate, known to be because of impairment of enterohepatic recirculation. Close monitoring is required.
Typhoid vaccine live: amoxicillin decreases the effects of this vaccine by pharmacodynamic antagonism. This combo presents heightened risk of life threatening interaction. It is contraindicated unless benefits outweigh the risks. Vaccine should be delayed until more than 3 days after antibiotic course is completed or complete vaccine >3 days before 1st antibiotic dose.
Acyclovir: increases levels of the other by decreasing renal clearance, the combination requiring close monitoring.
Allopurinol: allopurinol increases the risk of allergic or hypersensitivity reaction to amoxicillins.