Synergism is a useful mechanism where combining two or more drugs is more effective than each constituent used alone. Such combinations are also called supra-additive interactions. The drugs collectively enhance the final therapeutic effect by acting on different targets. Another advantage is that the low dose of each constituent drug is sufficient to achieve the desired effect. This helps reduce the duration of therapy and lower the adverse effects of these drugs.

Such synergistic combinations of antibiotics help reduce the minimum inhibitory concentration (MIC) of each by four-fold or more. Synergistic combinations of antibiotics are useful when an infectious agent is unknown or when pathogens have populations of varying drug sensitivity, such as Mycobacterium tuberculosis.

Combining trimethoprim and sulfamethoxazole represents one of the most effective treatments for urinary tract infections. This combination works by inhibiting the activity of two different enzymes involved in the folate pathway, which block the synthesis of purines, the building blocks of DNA. The unavailability of purines impedes DNA synthesis, which ultimately kills the bacteria. Other antibiotic combinations use cell wall-active drugs, such as penicillins, which enhance the permeability of drugs, combined with aminoglycosides, which act on intracellular targets, like 30S ribosomes. These combinations prove highly effective in treating such infections as bacterial endocarditis caused by Enterococcal endocarditis.

In addition, some drugs do not have their own effects but potentiate the effect of other drugs in combination. One useful interaction is between β-lactamase inhibitors such as clavulanic acid and β-lactamase susceptible penicillins. Antibiotics such as penicillins and cephalosporins have β-lactam rings. The bacterial β-lactamase enzyme hydrolyses these rings, making these drugs inactive and leading to the emergence of antibiotic resistance. Using β-lactamase inhibitors and such antibiotics helps overcome the resistance that these bacterial enzymes generate.