Sulfamethoxazole: What is it? Uses, Dosage, Interactions, Side Effects, Considerations and Precautions

It is an antibiotic prescribed for the treatment of malaria, chlamydial conjunctivitis, toxoplasmosis and urinary tract infections.

It is an antibacterial sulfonamide. It prevents the formation of dihydrofolic acid, a compound that bacteria must be able to survive.

Although it was once a very useful antibiotic, it is now almost obsolete as a single agent due to the development of bacterial resistance to its effects.

Sulfamethoxazole is now used mainly in combination with trimethoprim , a product known as Bactrim or Septra.

Sulfamethoxazole was approved by the FDA in 1961. But now, according to the FDA database, all generic and brand-name formulations of Sulfamethoxazole have been discontinued.

The combination of Sulfamethoxazole and Trimethoprim is used to treat infections such as urinary tract infections, middle ear infections (otitis media), bronchitis, traveler’s diarrhea , and shigellosis (bacillary dysentery).

This medicine is also used to prevent or treat pneumonia by Pneumocystis carinii or Pneumocystis carinii pneumonia (PCP), a severe form of pneumonia.

This type of pneumonia occurs more frequently in patients whose immune systems do not function normally, such as cancer patients, transplant patients, and patients with acquired immunodeficiency syndrome ( AIDS ).

The combination of Sulfamethoxazole and Trimethoprim creates an antibiotic. It works by eliminating bacteria that cause many types of infections.

This medication will not work for colds, flu or other virus infections and is only available with your doctor’s prescription.

Side effects

The common side effects of Sulfamethoxazole are:

  • Headache.
  • Sickness.
  • Vomiting
  • Eruption.

Sulfamethoxazole should be discontinued at the first appearance of a rash as the rash may become severe. Serious rashes include:

  • Stevens-Johnson syndrome (joint and muscle pain, redness, blistering and peeling skin).
  • Toxic epidermal necrolysis (difficulty in swallowing, peeling, redness, loosening and blistering of the skin).

Sulfamethoxazole therapy can also cause extensive sunburn after exposure to sunlight. Patients receiving Sulfamethoxazole should avoid excessive exposure to sunlight and should use a protector.

Uncommon side effects:

  • Hepatic injury.
  • Low white blood cell count.
  • Low platelet count ( thrombocytopenia ).

Sulfamethoxazole can form crystals in the urine that can damage the kidney and cause hemorrhage. It is important to drink extra fluids during therapy to prevent these side effects.

What is the right dose?

The dose of this medication will be different for each patient. Follow your doctor’s orders or the instructions on the label.

Sulfamethoxazole is usually taken two or three times a day, with or without food. It should be taken with 6 to 8 ounces of fluid to prevent crystals from forming in the urine. People with advanced kidney disease may require lower doses.

For the treatment of bacterial infections:

Adults and children weighing 40 kilograms (kg) or more: 800 milligrams (mg) of Sulfamethoxazole and 160 mg of Trimethoprim every 12 hours for 10 to 14 days. Your doctor can adjust this dose if necessary.

Children 2 months of age and older and weighing up to 40 kg: 40 milligrams (mg) per kilogram of body weight of Sulfamethoxazole and 8 milligrams (mg) per kilogram of body weight of trimethoprim, administered in two divided doses every 12 hours during 10 days.

Babies under 2 months of age: its use is not recommended.

For traveler’s diarrhea:

Adults: 800 milligrams (mg) of Sulfamethoxazole and 160 mg of Trimethoprim every 12 hours for 5 days.

Children 2 months of age and older: use and dosage should be determined by your doctor.

Children under 2 months of age: its use is not recommended.

Appropriate use

Take this medicine exactly as directed by your doctor. Do not take more, do not take it too often and do not take it for a longer time than prescribed by your doctor. Doing so may increase the possibility of side effects

The combination of Sulfamethoxazole and Trimethoprim is best taken with a full glass (8 ounces) of water. You should drink several extra glasses of water every day, unless your doctor tells you otherwise. Drinking extra water will help prevent some unwanted effects.

For patients taking oral liquid, use a specially marked measuring spoon or other device to measure each dose accurately.

Note: The average household spoon can not contain the correct amount of liquid.

To help heal your infection completely, continue using Sulfamethoxazole for the entire treatment period, even if you start to feel better after a few days. If you stop taking it too soon, your symptoms may return.

Interactions with other medications

Sulfamethoxazole may increase the anticoagulant effects of warfarin (Coumadin), possibly causing bleeding.

Sulfonamides such as Sulfamethoxazole can increase the metabolism (breakdown and elimination) of Cyclosporine (causing the loss of effectiveness of Cyclosporin) and may increase renal damage caused by Cyclosporin.

All sulfonamides can crystallize in the urine when the urine is acidic. Like Metenamine (Hiprex, Urex, Mandelamine) that causes acidic urine, it should not be used with sulfonamides.

The use of Sulfamethoxazole with any of the following medications is also not recommended:

The use of this medication with any of the following is generally not recommended, but may be necessary in some cases. If both medications are prescribed together, your doctor may change the dose or how often you use one or both.

  • Arsenic trioxide.
  • Azilsartan Medoxomil.
  • Candesartán Cilexetil.
  • Chloral hydrate.
  • Vaccine against cholera.
  • Aminolevulinic acid.
  • Phenytoin

Other interactions

Certain medications should not be used at or near the time of eating, as interactions may occur. Using alcohol or tobacco with certain Sulfamethoxazole can also cause interactions.

The use of this medication with any of the following is generally not recommended, but may be unavoidable in some cases.

If used together, your doctor may change the dose or how often you use this medication, or give special instructions about the use of food, alcohol, or tobacco.

  • Ethanol

Other medical problems

The presence of other medical problems may affect the use of this medication. Be sure to tell your doctor if you have any other problems, especially:

  • Alcohol abuse.
  • Folate deficiency (vitamin B9).
  • VIH about SIDA.
  • Renal disease.
  • Liver disease
  • Malabsorption syndrome (difficulty absorbing food in the body).
  • State of malnutrition (nutrition disorder): use with caution. You may have a greater chance of suffering serious side effects.
  • Megaloblastic anemia (caused by low levels of folic acid in the body).
  • Drug-induced thrombocytopenia (low level of platelets in the blood).
  • Kidney disease, severe.
  • Liver disease, serious.
  • Severe allergies
  • Thyroid problems: use with caution. It can make these conditions worse.

Important considerations

Is it safe to take Sulfamethoxazole during pregnancy or breastfeeding?

The effects on the fetus have not been adequately studied. Therefore, doctors can use them if the benefits are considered to outweigh the potential risks.

On the other hand, the use of sulfonamides in the short term (that is, close to the ninth month of pregnancy) can cause the bilirubin to move from the proteins in the baby’s blood.

The displacement of bilirubin can lead to a dangerous condition called Kernicterus in which bilirubin damages the brain. For this reason, sulfonamides should not be used near term delivery.

Sulfamethoxazole is excreted in breast milk and should not be used in women who are breastfeeding. Sulfonamides can cause Kernicterus in newborn infants.

How should I keep Sulfamethoxazole stored?

  • The tablets should be kept at room temperature, 15 C – 30 C (59 F – 86 F).
  • Store the medicine in a closed container, away from heat, moisture and direct light. Avoid freezing.
  • Keep out of children reach.
  • Do not keep obsolete medicine or medications that you no longer need. Ask your healthcare provider how you should get rid of any medication you do not use.

Did you forget a dose?

If you miss a dose of this medication, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule.


Tell your doctor if you have ever had an allergic or unusual reaction to this medicine or other medicines. Also inform your health care professional if you have any other type of allergy, such as food, dyes or preservatives.


Appropriate studies performed to date have not shown specific pediatric problems that limit the usefulness of Sulfamethoxazole and Trimethoprim in children 2 months of age or older.

However, due to the toxicity of the combination of Sulfamethoxazole and Trimethoprim, use in infants under 2 months of age is not recommended.


Appropriate studies to date have not shown specific problems that limit the usefulness of the combination of Sulfamethoxazole and Trimethoprim in the elderly.

However, elderly patients are more likely to have a folate deficiency, age-related kidney or liver problems, and may be more likely to experience unwanted side effects.

As for example severe skin rash, increase of potassium in the body or problems of blood coagulation or the immune system.

There may be a dose adjustment for elderly patients receiving a combination of Sulfamethoxazole and Trimethoprim.


Studies in women suggest that this medication poses a minimal risk to the baby when used during breastfeeding.


It is very important that your doctor check your progress on regular visits to make sure this medication is working properly.

Blood and urine tests may be needed to check for unwanted effects.

Using this medicine while you are pregnant can harm your unborn baby. Use an effective form of birth control to avoid getting pregnant.

If you think you have become pregnant while using the medication, tell your doctor immediately.

Very rarely, this medication has caused serious side effects. If you start having a rash, or if you think you are having a serious skin reaction, stop taking this medicine and call your doctor immediately.

Symptoms of a severe reaction may include a rash, a very pale or yellow skin color or skin with purple spots, along with a sore throat, fever, muscle pain, cough, and shortness of breath.

This medicine, especially if you are receiving high doses or for a prolonged period of time, can reduce the number of platelets in your body, which are necessary for the proper coagulation of the blood.

Because of this, you can bleed or get infections more easily. Talk to your doctor if you have questions about it.

This medicine can cause diarrhea and, in some cases, it can be serious. It may occur in 2 months or more after you stop taking this medication.

Do not take any medication to treat diarrhea without first talking to your doctor. If you have any questions or if the mild diarrhea continues or gets worse, check with your doctor.

If you have abdominal or stomach cramps, swelling, severe and watery diarrhea, which may also be bloody, nausea or vomiting, or unusual tiredness or weakness, may be symptoms of a severe intestinal infection.

Sulfamethoxazole can cause severe allergic reactions, including anaphylaxis . Anaphylaxis can be life-threatening and requires immediate medical attention.

Check with your doctor if you have a rash, itching, swelling of the face, tongue and throat, shortness of breath or chest pain after using the medicine.

It can also cause electrolyte problems, such as high levels of potassium in the blood (hyperkalemia) and low blood sodium (hyponatremia).

Tell your doctor immediately if you experience any of the following symptoms while using this medicine: confusion, weakness, muscle spasms, irregular heartbeat, numbness or tingling in your hands, feet or lips, or trouble breathing.

Before having medical tests, inform the doctor in charge that you are taking Sulfamethoxazole. The results of some tests may be affected by this medication.

Patients receiving anticonvulsant therapy (medicines to prevent seizures) may be at risk for a deficiency of folate (vitamin B9), which may increase the risk of side effects. Talk to your doctor if you have questions about it.