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, 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.

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 are based on the child’s age and weight. Your doctor will calculate the appropriate dose for your child.

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 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, including porphyria
• have severe reduction of kidney function
• are pregnant (unless, in the judgment of the doctor, the benefits outweigh the risks)
• are breast-feeding

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

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.

• abdominal pain
• constipation
• diarrhea
• dizziness
• dry mouth
• eye inflammation (e.g., redness, pain)
• gas
• hair loss
• headache
• heartburn
• increased urgency to urinate
• loss of appetite
• mouth pain
• mouth sores
• nausea
• ringing of the ears
• spinning sensation
• swelling or puffiness
• trouble sleeping
• unusual tiredness or weakness
• 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:

• chills
• difficulty controlling movements
• hallucinations (hearing or seeing things that aren’t there)
• increased sensitivity of skin to sunlight
• mood changes
• muscle and joint pain
• signs of clotting problems (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don’t stop bleeding, skin rash that appears as pinpoint reddish or purple spots)
• signs of depression (e.g., poor concentration, changes in weight, changes in sleep, decreased interest in activities, thoughts of suicide)
• signs of heart problems (e.g., fast heart rate, chest pain, shortness of breath)
• 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,
• symptoms of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• symptoms of low blood sugar (e.g., cold sweat, cool pale skin, headache, fast heartbeat, weakness
• symptoms of vaginal yeast infection (e.g., white vaginal discharge, vaginal itching)
• tingling and numbness in the hands and feet
• uncontrolled tremor or shaking
• vision changes
• white patches in the mouth or on the tongue

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

• diarrhea (watery and severe; may also be bloody)
• 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 pancreatitis (e.g., abdominal pain on the upper left side, back pain, nausea, fever, chills, rapid heartbeat, swollen abdomen)
• 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
• symptoms of DRESS syndrome (e.g., fever, severe skin rash, peeling skin, abnormal blood and liver function tests)

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.

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.

Bacterial resistance: Misuse of an antibiotic such as trimethoprim-sulfamethoxazole may lead to the growth of resistant bacteria that will not be killed by the antibiotic. If this happens, the antibiotic may not work for you in the future. Although you may begin to feel better early in your course of treatment with trimethoprim-sulfamethoxazole, you need to take the full course exactly as directed to finish ridding your body of the infection and to prevent resistant bacteria from taking hold. Do not take this or other antibiotics to treat a viral infection such as the common cold; antibiotics do not kill viruses, and using them to treat viral infections can lead to the growth of resistant bacteria.

Blood counts: When taken for long periods of time, this medication can decrease the number of neutrophils (a type of white blood cell that helps fight infection), red blood cells (which carry oxygen), and platelets (which help your blood to clot). Your doctor will do blood tests to monitor this. If you notice any signs of infection (e.g., fever, chills, or sore throat) or unusual bleeding or bruising, contact your doctor immediately.

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.

Fluid and electrolyte balance: Trimethoprim - sulfamethoxazole may cause the levels of electrolytes, such as potassium and sodium, in the blood to change while taking this medication. If you experience symptoms of fluid and electrolyte imbalance such as muscle pains or cramps; achy, stiff, or uncoordinated muscles; tiredness; or weakness; contact your doctor as soon as possible.

Glucose-6-phosphate dehydrogenase enzyme deficiency (G6PD): Trimethoprim – sulfamethoxazole may cause people lacking the G6PD enzyme to experience excessive and early breakdown of red blood cells. If you have this condition, discuss with your doctor how this condition may affect the dosing and effectiveness of this medication.

Hypersensitivity reaction: Rarely, a severe allergic reaction called hypersensitivity syndrome or Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS syndrome) has been reported by some people using trimethoprim – sulfamethoxazole. This reaction can affect organ function and should be treated as an emergency. Get immediate medical attention if you have symptoms of a severe allergic reaction, including fever, swollen glands, yellowing of the skin or eyes, or flu-like symptoms with skin rash or blistering.

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.

Liver function: People taking this medication may have changes in liver function that produce abnormal liver test results. If you have reduced liver function, 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.

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.

Porphyria: Trimethoprim – sulfamethoxazole may cause attacks of a condition called acute porphyria, a hereditary condition that affects the production of heme in the body. Heme is an important part of red blood cells, which carry oxygen throughout the body. If you have porphyria, discuss with your doctor whether any special monitoring is needed.

Sensitivity to sunlight: This medication may increase the sensitivity of the skin to sunlight, increasing the risk of sunburn. Avoid exposure to excessive sunlight, including sunlamps and tanning beds, and use sunblock with a minimum SPF of 15. 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 breast-feeding 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.

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

• acetylsalicylic acid (ASA)
• amantadine
• androgens (e.g.,  testosterone)
• angiotensin converting enzyme inhibitors (ACEIs; captopril, enalapril, ramipril)
• angiotensin receptor blockers (ARBs; e.g., candesartan, irbesartan, losartan)
• azathioprine
• BCG
• cholera vaccine
• cyclosporine
• dapsone
• diabetes medications (e.g., acarbose, canagliflozin, glyburide, insulin, linagliptin, lixisenatide, metformin, repaglinide, rosiglitazone)
• digoxin
• disopyramide
• eplerenone
• folic acid
• hormonal contraceptives (e.g., birth control pills)
• lamivudine
• leucovorin
• local anaesthetics (medications to numb skin or gums; lidocaine, prilocaine, xylocaine)
• memantine
• mercaptopurine
• methotrexate
• methoxsalen
• mifepristone
• monoamine oxidase inhibitors (MAOIs; e.g., moclobemide, phenelzine, rasagiline, selegiline, tranylcypromine)
• pegvisomant
• phenytoin
• procainamide
• quinine
• quinolone antibiotics (e.g., ciprofloxacin, norfloxacin, ofloxacin)
• rifampin
• St. John's wort
• selective serotonin reuptake inhibitors (SSRIs; e.g., citalopram, fluoxetine, paroxetine, sertraline)
• sodium picosulfate
• somatostatin-like medications (e.g., lanreotide, octreotide, pasireotide)
• spironolactone
• other sulfonamide antibiotics ("sulfas"; e.g., sulfisoxazole, sulfadiazine )
• sunitinib
• tramadol
• typhoid vaccine
• warfarin
• zidovudine

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.