You use two drugs (sulfisoxazole and trimethoprim) to treat a bacterial infection and notice that bactericidal activity is enhanced. this is known as:

Antagonsitic effect/interaction/response

In order to combat antiobiotic resistance, and to possibly enhance the activity of antibiotics, they are sometimes used in combinations during treatment. However, three possible responses or effects can manifest.

First is antibiotic synergy, where the combined effect of the antibiotics enhances the activity/potency of the treatment compared to when the antibiotics are administered singly.

The effect is also distinguished from another type of response, which is additive effect, where the combined effect of the antibiotics is more or less equal to the combined activity/potency of each of the antibiotic when applied singly. Antibiotic synergy results in even greater enhancement of the activity of the combined antibiotics compared to additive effect.

Lastly, there is the antagonistic effect or response, where the combined effect of the antibiotics results in the weakening of the potencies of the antibiotics relative to the combined (additive effect) potencies of each of the antibiotics.