Trimethoprim is an antibiotic that inhibits bacterial dihydrofolate reductase, disrupting folate synthesis and ultimately hindering DNA synthesis in susceptible organisms. It is clinically significant for treating urinary tract infections and respiratory infections due to its effectiveness against common pathogens like Escherichia coli. A key association is its use in combination with sulfamethoxazole, enhancing efficacy and reducing resistance development, but it may cause adverse effects such as hyperkalaemia.