The drug should not be used alone for empiric therapy in seriously ill patients if there is a possibility that the infection may be caused by gram-positive bacteria or if a mixed aerobic-anaerobic bacterial infection is suspected. In such infections, another anti-infective agent effective against the suspected, potentially aztreonam-resistant organism should initially be used concomitantly. Aztreonam has been used safely and effectively in conjunction with an aminoglycoside, clindamycin, erythromycin, metronidazole, a penicillin, or vancomycin.