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sulfamethoxazole/trimethoprim (Bactrim) Side Effects & Dosage

What is sulfamethoxazole and trimethoprim, and how does it work (mechanism of action)?

Bactrim is a combination of two synthetic
(man-made) antibiotics, sulfamethoxazole and trimethoprim. Both drugs reduce the
ability of some bacteria to utilize folic acid for growing. Sulfamethoxazole is
an anti-bacterial sulfonamide, a "sulfa" drug. It disrupts the production of dihydrofolic acid while trimethoprim disrupts the production of tetrahydrofolic
acid. Dihydrofolic acid and tetrahydrofolic acid are forms of folic acid that
bacteria and human cells use for producing proteins. Trimethoprim inhibits
production of tetrahydrofolic acid by inhibiting the enzyme responsible for
making tetrahydrofolic acid from dihydrofolic acid. By combining both drugs, two
important steps required in the production of bacterial proteins are
interrupted, and the combination is more effective than either drug alone.
Bactrim was approved by the FDA in 1973.

What brand names are available for sulfamethoxazole and trimethoprim?

Bactrim, Bactrim DS, Sulfatrim Pediatric

Is sulfamethoxazole and trimethoprim available as a generic drug?


Do I need a prescription for sulfamethoxazole and trimethoprim?


What are the uses for sulfamethoxazole and trimethoprim?

Sulfamethoxazole/trimethoprim is used for treating
infections due to susceptible bacteria. Examples include
urinary tract
infections, flares of chromic bronchitis due to bacteria,
middle ear infections,
for prevention of infections due to pneumococcus in organ transplant recipients,
for the treatment or prevention of
Pneumocystis carinii pneumonia,
and prevention of toxoplasma encephalitis in patients with

What are the side effects of sulfamethoxazole and trimethoprim?

Common side effects of sulfamethoxazole/trimethoprim are:

Other side effects include:

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What is the dosage for sulfamethoxazole and trimethoprim?

The recommended adult dose for urinary tract infections is one
double strength tablet (Bactrim DS, Septra DS) or two single strength tablets
every 12 hours for 10 to 14 days.

Flares of chronic bronchitis are treated with
a similar regimen for 14 days.

Sulfamethoxazole/trimethoprim should be taken
with 6 to 8 ounces of liquid to prevent crystals from forming in the urine.
Persons with advanced kidney disease may require lower doses.

Which drugs or supplements interact with sulfamethoxazole and trimethoprim?

Sulfamethoxazole/trimethoprim can enhance the
blood-thinning effects of warfarin (Coumadin), possibly leading to bleeding.
Sulfonamides such as sulfamethoxazole can increase the metabolism (break-down
and elimination) of cyclosporine (causing loss of effectiveness of
cyclosporine), and can add to the kidney damage caused by cyclosporine.

sulfonamides can crystallize in urine when the urine is acidic. Since methenamine (Hiprex, Urex, Mandelamine) causes acidic urine, it should not be
used with sulfonamides.

Blood levels of phenytoin (Dilantin) may be increased by
treatment with sulfamethoxazole/trimethoprim. This may lead to side effects
associated with phenytoin (Dilantin,
Dilantin-125) such as
dizziness, and reduced attention.

Sulfamethoxazole/trimethoprim also may increase blood levels of digoxin (Lanoxin)
and possibly lead to serious toxic effects. Anemia, due to a reduction in folic
acid, can occur in persons receiving sulfamethoxazole/trimethoprim in
combination with:

Increased blood levels of potassium may occur when sulfamethoxazole/trimethoprim is combined with ACE inhibitors.


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Is sulfamethoxazole and trimethoprim safe to take if I’m pregnant or breastfeeding?

Use of sulfonamides may cause bilirubin to be displaced
from proteins in the infant's blood. Displacement of bilirubin can lead to
jaundice and a dangerous condition called kernicterus in the infant. For this
reason, sulfamethoxazole/trimethoprim should not be used near term (late in
pregnancy) among women.

Sulfamethoxazole/trimethoprim should not be used by
nursing mothers because sulfamethoxazole is excreted in milk and can cause

What else should I know about sulfamethoxazole and trimethoprim?

What preparations of sulfamethoxazole and trimethoprim are available?

Tablets: 160 mg trimethoprim and 800 mg
sulfamethoxazole (Bactrim DS, Septra DS); 80 mg trimethoprim and 400 mg
sulfamethoxazole (Bactrim; Septra).

How should I keep sulfamethoxazole and trimethoprim stored?

The tablets should be kept at room temperature, 15 C -30 C
(59 F – 86 F).


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