Trimethoprim (TMP) is an antibiotic that is used to treat certain types of urinary tract, ear and intestinal infections that are caused by bacteria. It is is commonly included in combination antibiotics with Sulfamtheoxazole, Dapsone and other Sulfonamide antibiotics.

Trimethoprim inhibits the ability of bacteria to synthesize vitamin B9 (Folate) by a mechanism that complements the antibacterial activity of Sulfonamide antibiotics.

Trimethoprim is occasionally used alone as a treatment for acne. There is limited clinical research into the efficacy of Trimethoprim alone as an acne treatment, but some acne patients have reported positive results with this medication.

Laboratory testing indicates that the acne-causing P. acnes bacterium tends to be moderately susceptible to Trimethoprim, but Trimethoprim-resistant P. acnes are becoming more common in some places.

The combination of Trimethoprim plus Sulfamethoxazole (CoTrimoxazole) is a more common and effective acne treatment. Oral Cotrimoxazole can be a very effective treatment for some individuals with moderate to severe inflammatory acne symptoms. CoTrimoxazole is a popular choice for acne patients who have not responded to other types of antibiotics (eg. Doxycycline, Minocycline, Erythromycin).