Combination of antimicrobial are sometimes used to treat infection, but care must be taken when selecting the combination because some drugs will counteract the effects of others. For example bacteria static drugs interfere with the bactericidal effects of penicillin because penicillin only kills actively divided cells. The combination of one drugs that interfere with activities of others is called antagonist. In contrast, drug that are rare more effective when taken together are called synergistic. For example, the combination of penicillin and streptomycin is more lethal to bacteria than in the summer of their individual activities because penicillin interfere with cell wall synthesis which, in turns allow streptomycin to more easily enter cells.

The synergistic effects sometimes allow lower doses of toxic drug to be used. Drug combination that are neither antagonistic nor synergistic are called additives. An additives combination results in total effects that is neither greater nor less than the sum of the effects of the two individuals drugs.

Other examples are when taking the combination of rifampin and isoniazid, which are both used to treat tuberculosis.