Trimethoprim is an antibiotic used to treat bladder (cystitis) and urinary tract infections (UTIs). It is included on the World Health Organization's List of Essential Medicines.
Taking a three day course of Trimethoprim will clear most bladder and urinary tract infections. However, if your symptoms have not improved once you have completed your medication, you should seek advice from your GP.
Most cases of cystitis or UTIs are caused by E.coli, which originates in your gut. Trimethoprim does not kill E.coli. Instead, it stops bacterial DNA synthesis, meaning that the bacteria cannot replicate. This allows your immune system to destroy the infecting bacteria until they are eradicated and your infection is resolved.