Synergism describes the interaction of two or more drugs whose combined effect is greater than the sum of the effects of these drugs used alone. Synergistic drugs help treat various cancers, relieve excruciating pain, and eliminate infectious agents. Usually, the constituent drugs act on different targets, enhancing the final therapeutic effect. One of the best examples is the antibiotic combination of trimethoprim-sulfamethoxazole used to treat urinary tract infections. Sulfamethoxazole inhibits dihydropteroate synthase and prevents para-Aminobenzoic acid's conversion to dihydrofolic acid. Trimethoprim selectively inactivates bacterial dihydrofolic acid reductase to block tetrahydrofolic acid formation. Such sequential inhibition of the folate synthetic pathway prevents DNA synthesis in bacteria. Another effective combination of penicillin and aminoglycosides treats several bacterial infections. Penicillin inhibits the synthesis of peptidoglycans, a key component of bacterial cell walls, making the cell walls porous. This increases the uptake of aminoglycosides, which inactivates the 30S ribosomal subunit, blocking translation.