Health Library Sulfonamide and Trimethoprim for Urinary Tract Infections (UTIs)From Healthwise

Sulfamethoxazole is an antibiotic that comes in tablet or syrup form to take by mouth (orally). This is a sulfa drug; do not take it if you have an allergy to sulfa drugs.

Trimethoprim is an antibiotic that comes in pill or tablet form that you take by mouth (orally).

These antibiotics kill the bacteria that commonly cause urinary tract infections (UTIs).

Sulfamethoxazole with trimethoprim are combinations of two different antibiotics and are the most common antibiotics to treat uncomplicated urinary tract infections. These are infections that are not caused by a blockage or structural problem or some other medical condition that affects urinary function, such as diabetes. They also prevent recurrent urinary tract infections.

Taking sulfamethoxazole with trimethoprim for 3 days usually is effective treatment for uncomplicated bladder infections.¹

Common side effects are:

• Increased sun sensitivity.
• Rashes.
• Upset stomach.

Rare but serious side effects include destruction of red blood cells and severe skin rashes.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

If you have frequent uncomplicated UTIs, your doctor may write a standing prescription for a sulfonamide/trimethoprim combination to take if you develop UTI symptoms. Follow your doctor's instructions for taking the medicine and monitoring your symptoms.

These medicines can be given for 1, 3, or 7 days for an uncomplicated bladder infection. A 3-day course of therapy appears to be just as effective as a 7-day course of therapy in most people.

Do not take sulfonamides if you are allergic to a sulfa drug.

Take sulfamethoxazole with trimethoprim 1 to 2 hours before or after meals. Take them with a full glass of water, and drink several additional glasses of water every day.

These medicines are more effective when the urine is less acidic. Avoid foods and liquids that make urine more acid, such as vitamin C supplements and fruits high in vitamin C, while taking these medicines.

Do not take this medicine during the last 3 months of pregnancy.

Sulfonamides can interact with coumarin anticoagulants, such as warfarin (Coumadin), upsetting the level of the medicine in the body.

Antibiotic resistance

Be sure to take all of the medicine your doctor has prescribed, even if you are feeling better. If you do not take all of your antibiotics as prescribed, the infection may return. Not taking the full course of antibiotics also encourages the development of bacteria that are resistant to antibiotics. This makes antibiotics less effective and bacterial infections more difficult to treat.

Antibiotic-resistance among bacteria that cause UTIs has increased steadily in recent decades. You and your doctor may have to try different antibiotics, and different combinations of antibiotics, to find the right medicine that will kill the bacteria causing your UTI. Before you start a new antibiotic, your doctor will use results of urine tests to help select a medicine.

Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.

Citations

1. Choice of antibacterial drugs (2004). Treatment Guidelines From the Medical Letter, 2(19): 13–26.

Author:	Kathe Gallagher, MSW	Last Updated: June 15, 2007
Medical Review:	E. Gregory Thompson, MD - Internal Medicine Philip Belitsky, MD, FRCSC - Urology
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