action of the combination is significantly less than the sum of the actions  of each component. Most commonly cited is “bacteriostatic drug inhibits action of bacteriocidal”. Usually, bacteriostatic activity is sufficient for cure and you only you only waste money. Antagonism is only evident (clinically) when the patient is dependent on the antimicrobial for survival or cure.

• Two antimicrobials may compete for the same binding site. This is unlikely to have a clinical effect as each site can only be inhibited once. You just wasted some more
• One antimicrobial may inhibit of cell permeability to a second antimicrobial. Frequency and significance of this is uncertain.
• One antimicrobial may cause “derepression” of resistance enzymes for a second. Administration of older beta-lactam can increase production of beta-lactams directed against new cephalosporins.