This is called maintenance therapy or secondary prophylaxis. Some people are allergic to Septrin and develop reactions such as an itchy red rash, sometimes with fever. In affected people, this usually occurs during the second week of taking the drug. In rare cases these reactions are extremely serious, so they should be reported to your doctor at once. It can also affect people using Septrin for prophylaxis.
Some people also experience a shortage of white blood cells.
These blood side-effects are more common if you are also taking certain medications such as AZT or ganciclovir. Some people taking Septrin also experience liver problems. Blood tests can monitor both your blood cell levels and your liver function. People with kidney damage may need to take lower doses. Other possible effects of Septrin include reduced levels of folate in the body, which can be corrected by a prescription of folinic acid, and a decrease in the effectiveness of oral contraceptives.