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Medical Uses and Side Effects of Sulfamethoxazole/Trimethoprim

Medical Uses and Side Effects of Sulfamethoxazole/Trimethoprim

Sulfamethoxazole/Trimethoprim is a combination antibiotic that is used for the treatment of infections that are caused by certain bacteria. This HealthHearty write-up provides information on the medical uses and side effects of sulfamethoxazole/trimethoprim.
Smita Pandit
Did You Know?
Sulfamethoxazole/Trimethoprim is a combination antibiotic drug that is placed in the World Health Organization's List of Essential Medicines. Sulfamethoxazole is used in combination with trimethoprim, on account of the development of bacterial resistance to the effects of the former.
Marketed under the brand names such as Bactrim, Cotrim, Septra, Sulfatrim, etc., the combination of sulfamethoxazole and trimethoprim (TMP-SMZ) is believed to be very effective in the treatment of bacterial infections. It was in 1973 that TMP-SMZ was approved for use as a combination antibiotic in the United States. This drug is classified under the class of drugs called sulfonamides. Antibiotics can be bactericidal (the ones that destroy bacteria) or bacteriostatic (the ones that inhibit the growth of bacteria by interfering with the synthesis of proteins and replication of the DNA that is required for bacterial growth) in nature. It must be noted that some bacteriostatic antibiotics might act as bactericidal antibiotics in high concentrations, whereas some bactericidal antibiotics might act as bacteriostatic antibiotics in low concentrations. Sulfonamides and Trimethoprim come under this category.

The main reason behind the use of combination antibiotic drugs, such as Bactrim, is to treat cases where the exact cause or the causal bacterial strain has not been identified. Moreover, their use also prevents the development of resistant strains. Last but not the least, antibiotic synergism helps in the prompt treatment and recovery, as the effects of using a combination of antibiotics are more beneficial than using the individual antibiotics. Bactrim is prescribed for the treatment of a wide range of bacterial infections. It might also be prescribed for the prevention of infections caused by opportunistic bacteria or parasites. Opportunistic pathogens are disease-causing microbes that don't cause infections in healthy individuals, but lead to infections in individuals who are immunocompromised.
Mechanism of Action

Bacteria need to make new DNA, RNA, and proteins in order to replicate. This antibiotic binds to the ribosomes in the bacteria, thereby interfering with the synthesis of protein or nucleic acid. In this combination drug, sulfamethoxazole competes with para-aminobenzoic acid (PABA), thereby inhibiting bacterial synthesis of a folic acid derivative called dihydrofolic acid. Dihydrofolic acid can be converted to tetrahydrofolic acid by an enzyme called dihydrofolate reductase. Trimethoprim blocks the production of tetrahydrofolic acid by binding to this enzyme. Thus, sulfamethoxazole and trimethoprim obstruct the process of biosynthesis of nucleic acids and proteins that are essential to bacteria.

What is Sulfamethoxazole-Trimethoprim Used for?

Every year, more than 8 million prescriptions are being filled for TMP-SMZ. Bactrim (Oral) is available in the form of tablets, as well as a suspension. It is available as tablets with each tablet containing 400 mg sulfamethoxazole and 80 mg trimethoprim. In case of Bactrim-DS (double strength) tablets, each tablet contains 800 mg sulfamethoxazole and 160 mg trimethoprim. While treating any bacterial infection, it's essential to check for antibiotic susceptibility or the susceptibility of the bacterium to the antibiotic. It has been observed that the following bacteria are susceptible to Bactrim:

Escherichia coli (including enterotoxigenic strains that cause traveler's diarrhea)
Enterobacter species
Streptococcus pneumoniae
Shigella flexneri
Shigella sonnei
Staphylococcus (including MRSA)
Klebsiella species
Haemophilus influenzae
Morganella morganii
Proteus mirabilis
Proteus vulgaris
Pneumocystis jiroveci

This combination drug is mainly prescribed for the treatment of the following medical conditions:

Urinary tract infections
Otitis media
Traveler's diarrhea
Pneumocystis carinii pneumonia (PCP)


It is extremely essential to inform your healthcare provider about:

Any drug reactions that you may have experienced against sulfonamides (including sulfamethoxazole), trimethoprim, or any other antibiotics.
Pre-existing medical conditions (especially kidney/liver disease, AIDS, porphyria, G6PD deficiency, folate deficiency, asthma, allergies, etc.)
Any prescription drug, OTC medication/nutritional supplements that you are currently taking
Conception while taking sulfamethoxazole/trimethoprim

Pregnant women and nursing mothers must avoid taking this drug due to the lack of data regarding its effects. Animal studies reveal evidence of teratogenicity, but there's no controlled data regarding its effect in humans. It is placed in the Pregnancy category C/D by the FDA. It is believed that this drug can increase the risk of kernicterus in the newborn. This drug is also excreted into human milk. Thus, it must be used by nursing mothers, only if recommended by their healthcare provider.

Side Effects

This drug should be taken only for the prescribed duration of time in the doses recommended by the healthcare provider. Adverse effects can occur if one does not follow the guidelines. Side effects can occur in case of an allergic reaction. Medical help must be immediately sought if one experiences the following symptoms:

Difficulty in breathing
Swelling of the face, lips, and throat

Other side effects include:

Sore throat
Stomach pain
Loss of appetite
Blistering, peeling, and red skin rash
Easy bruising or bleeding
Bloody or watery diarrhea
Dark urine
Clay-colored stools
Pale skin
Muscle pain or weakness
On a concluding note, sulfamethoxazole/trimethoprim combination should be taken in the prescribed doses for the duration recommended by the doctor. In case of a missed dose, take the tablet as soon as possible. However, don't take a double dose if it is almost time for the next dose. It's best to store the drug at room temperature, keeping it away from direct light, heat, or moisture. Make sure that it is not within the reach of children. Do report any untoward effects that you experience after the consumption of the drug at the earliest.

Disclaimer: The information provided in this article is solely for educating the reader. It is not intended to be a substitute for the advice of a medical expert.