The efficacy of oral contraceptives may be impaired under concomitant administration of Benzylpenicillin sodium, which may result in unwanted pregnancy. Women taking oral contraceptives should be aware of this and should be informed about alternative methods of contraception.
There is reduced excretion of methotrexate (and therefore increased risk of methotrexate toxicity) when used with Benzylpenicillin sodium.
Probenecid inhibits tubular secretion of Benzylpenicillin sodium and so may be given to increase the plasma concentrations.
Penicillins may interfere with: Urinary glucose tests; Coomb's tests; Tests for urinary or serum proteins; Tests which use bacteria e.g. Guthrie test.