Nalidixic acid is given to treat urine infections. A urine infection is often called a urinary tract infection (UTI) by doctors. Most urine infections are caused by bacteria that come from the bowel.The bacteria thrive in urine and multiply quickly to cause an infection.Nalidixic acid works by stopping the bacteria which are the cause of the infection from multiplying.. 0. 0. Nalidixic acid is given to treat urine infections. It is important to complete the full course of treatment unless you are told otherwise. This is to prevent the infection from coming back. The most common side-effects are feeling sick, diarrhoea, feeling dizzy and headache.