Patients who are hypersensitive to penicillin. Long-term administration may cause development of non-susceptible bacteria. May interfere w/ results of some lab tests, causing false +ve reaction for glycosuria (methods of Benedict, Fehling & Clinitest). False +ve result may occur in direct Coombs test. Concomitant use w/ other nephrotoxic drug (eg, kanamycin, streptomycin, colistin, viomycin, polymyxin, neomycin, gentamycin). Reduce dose according to renal function test in case of pronounced renal insufficiency. Pregnancy. Discontinue breastfeeding or discontinue/abstain from therapy considering the benefit of breastfeeding for the child & the benefit of therapy for the woman. Childn <3 mth.