How does this medication work? What will it do for me?

Trimethoprim and sulfamethoxazole both belong to the class of medications called antibiotics. This combination of antibiotics is used together to treat infections caused by certain bacteria. It is most commonly used to treat infections of the bladder, ear, sinus, skin, intestine, and lung. It works by killing the bacteria that cause these infections.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

What form(s) does this medication come in?

Each round, white tablet, one side convex, scored and engraved "APO 400-80" and the other side flat and plain, contains sulfamethoxazole 400 mg and trimethoprim 80 mg. Nonmedicinal ingredients: methylcellulose, croscarmellose sodium, magnesium stearate, and colloidal silicon dioxide.

Apo-Sulfatrim DS
Each white, capsule-shaped tablet with one side scored and engraved "APO DS" contains sulfamethoxazole 800 mg and trimethoprim 160 mg. Nonmedicinal ingredients: methylcellulose, croscarmellose sodium, magnesium stearate, and colloidal silicon dioxide.

Apo-Sulfatrim Pediatric
Each round, white, flat-faced tablet with bevelled edges contains sulfamethoxazole 100 mg and trimethoprim 20 mg. Nonmedicinal ingredients: methylcellulose, croscarmellose sodium, magnesium stearate, and colloidal silicon dioxide.

Apo-Sulfatrim Oral Suspension
Each 5 mL of pink, cherry-flavoured liquid suspension contains sulfamethoxazole 40 mg and trimethoprim 8 mg. Nonmedicinal ingredients: purified water, sodium cyclamate, carboxymethylcelluose, microcrystalline cellulose, sorbitol solution, glycerin, polysorbate, armarnth dye, FD&C Yellow 6, and artificial cherry flavour.

How should I use this medication?

The usual recommended dose of trimethoprim – sulfamethoxazole for adults and children over 12 years of age is 2 regular-strength tablets or 1 double-strength tablet taken twice a day. It should be taken in the morning and evening and may be taken with or without food. Taking the medication with food will reduce the risk of stomach upset.

Children's doses vary according to age and are usually given in the liquid form. Use an oral syringe to measure each dose of the liquid, as it gives a more accurate measurement than household teaspoons. Take this medication with a full 8-ounce glass of water. Drink several additional glasses of water every day, unless otherwise instructed by your doctor.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. Finish all of this medication, even if you start to feel better. If you miss a dose, take it as soon as possible and continue on with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue on with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not dispose of medications in wastewater (e.g. down the sink or in the toilet) or in household garbage. Ask your pharmacist how to dispose of medications that are no longer needed or have expired.

Who should NOT take this medication?

Do not take this medication if you:

  • are allergic to trimethoprim, sulfamethoxazole, or any ingredients of this medication
  • are allergic to sulfonamide medications
  • have certain types of liver disease
  • have certain types of blood disorders
  • have severe reduction of kidney function
  • are pregnant (unless, in the judgement of the doctor, the benefits outweigh the risks)
  • are a breast-feeding mother

Do not give this medication to children less than 2 months of age.

What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • diarrhea
  • dizziness
  • headache
  • loss of appetite
  • mouth sores
  • nausea
  • vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • increased sensitivity of skin to sunlight

Check with your doctor immediately if any of the following side effects occur:

  • cough
  • irregular heart beat
  • itching
  • muscle and joint pain
  • signs of kidney problems (e.g., decreased urination, nausea, vomiting, swelling of the feet and ankles)
  • signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
  • symptoms of too much potassium in the body (e.g., muscle fatigue, weakness, difficulty moving, abnormal heart rhythms, nausea)
  • skin rash
  • unusual bleeding or bruising
  • unusual tiredness or weakness
  • white patches in the mouth or on the tongue

Stop taking the medication and seek immediate medical attention if any of the following occur:

  • bluish fingernails, lips, or skin
  • depression
  • diarrhea (watery and severe; may also be bloody)
  • hallucinations
  • seizures (convulsions)
  • shortness of breath
  • signs of meningitis not caused by infection (e.g., headache [severe], throbbing, or with stiff neck or back)
  • signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)
  • signs of a severe skin reaction such as blistering, peeling, a rash covering a large area of the body, a rash that spreads quickly, or a rash combined with fever or discomfort

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Antibiotic-related diarrhea: As with other antibiotics, trimethoprim – sulfamethoxazole can cause a severe form of diarrhea associated with a condition known as pseudomembranous colitis. If you develop severe diarrhea while taking (or within a few weeks of taking) this medication, contact your doctor.

Drowsiness/reduced alertness: Trimethoprim – sulfamethoxazole may cause some people to become dizzy. Determine your response to this medication before engaging in potentially dangerous activities such as driving or operating machinery.

Kidney function: Kidney disease or reduced kidney function may cause this medication to build up in the body, causing side effects. Trimethoprim – sulfamethoxazole may also cause decreased kidney function. If you have reduced kidney function or kidney disease, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Medical conditions: If you have liver or kidney damage, urinary obstruction, blood disorders, allergies, or bronchial asthma, discuss with your doctor the benefits and risks of this medication.

Symptoms such as rash, sore throat, fever, pallor, difficulty breathing, or jaundice (yellowing of eyes or skin) may be early indications of rare but severe reactions. Anyone experiencing these symptoms after starting this medication should contact their doctor immediately.

Other infections: The prolonged or repeated use of antibiotics may occasionally result in organisms not killed by the antibiotic to overgrow. This may result in conditions such as yeast infections. If you experience new symptoms of infection or find that the infection is not improving with correct use of this medication, contact your doctor.

Sensitivity to sunlight: This medication may increase the sensitivity of the skin to sunlight, increasing the risk of sunburn. If you notice any unusual skin rash or peeling, contact your doctor immediately.

Skin rash: Trimethoprim – sulfamethoxazole may cause skin rash or itchiness with or without a rash. Rarely, people taking trimethoprim – sulfamethoxazole experience a severe skin reaction that can be life-threatening. If you experience a rash that gets worse, develops into blisters or sores on the lips or eyes, or covers a large area of the body, contact your doctor immediately.

Pregnancy: This medication should not be used during pregnancy unless, in the opinion of your doctor, the benefits outweigh the risks. Pregnant women who take this medication should consider taking supplemental folic acid.

Breast-feeding: Both trimethoprim and sulfamethoxazole pass into breast milk. If you are a breast-feeding mother and are taking trimethoprim – sulfamethoxazole, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

Children: This medication is not recommended for children younger than 2 months of age.

Seniors: Seniors may be at increased risk of developing side effects when taking this medication. Lower doses may be required.

What other drugs could interact with this medication?

There may be an interaction between trimethoprim – sulfamethoxazole and any of the following:

  • alfuzosin
  • amantadine
  • amiodarone
  • anagrelide
  • angiotensin converting enzyme inhibitors (ACEIs; captopril, enalapril, ramipril)
  • angiotensin receptor blockers (ARBs; e.g., candasartan, irbesartan, losartan)
  • antiemetics (e.g., granisetron, ondansetron)
  • antipsychotics (e.g., chlorpromazine, clozapine, haloperidol, olanzepine, quetiapine, risperidone)
  • aprepitant
  • azathioprine
  • "azole" antifungals (e.g., itraconazole, ketoconazole, voriconazole)
  • barbiturates (e.g., butalbital, pentobarbital, phenobarbital)
  • BCG
  • bosentan
  • carbamazepine
  • carvedilol
  • celecoxib
  • cisapride
  • chlorpropamide
  • citalopram
  • cyclosporine
  • dapsone
  • deferasirox
  • dexamethasone
  • diabetes medications (e.g., chlorpropamide, glipizide, glyburide, metformin, nateglinide, rosiglitazone)
  • digoxin
  • disopyramide
  • dofetilide
  • domperidone
  • dronedarone
  • eplerenone
  • enzalutamide
  • fenofibric acid
  • fingolimod
  • flecainide
  • fluorouracil
  • fluoxetine
  • fluvastatin
  • gemfibrozil
  • HIV non-nucleoside reverse transcriptase inhibitors (NNRTIs; e.g., delaviridine, efavirenz, etravirine, nevirapine)
  • lamivudine
  • leflunomide
  • leucovorin
  • metformin
  • methotrexate
  • mercaptopurine
  • mestranol
  • metformin
  • milk thistle
  • montelukast
  • nicardipine
  • nilotinib
  • omeprazole
  • oxcarbazepine
  • peginterferon Alfa-2b
  • phenytoin
  • piroxicam
  • primidone
  • procainamide
  • quinidine
  • quinine
  • quinolone antibiotics (e.g., ciprofloxacin, norfloxacin, ofloxacin)
  • rifabutin
  • rifampin
  • romidepsin
  • St. John's wort
  • spironolactone
  • other sulfonamide antibiotics ("sulfas"; e.g., sulfisoxazole, sulfadiazine )
  • tamoxifen
  • tetrabenazine
  • ticagrelor
  • trazodone
  • warfarin
  • varenicline
  • zafirlukast

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.