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What is Bactrim (sulfamethoxazole/trimethoprim)?

Bactrim is an antibiotic that carries a combination of sulfamethoxazole and trimethoprim. They're both antibiotics that treat and stop the growth of bacterial infections.

What does Bactrim treat?

Doctors usually prescribe Bactrim for bacterial infections, including:

  • Urinary tract infection

  • Middle ear infections in children

  • Chronic bronchitis flare-ups in adults

  • Shigellosis

  • Traveler's diarrhea in adults

  • Pneumonia prevention for people with weakened immune systems

  • Bacterial infection protection for people receiving chemotherapy

Bactrim does not work against infections caused by viruses, such as the common cold and flu. It also isn’t effective against streptococcal bacteria infections.

Bactrim is only available with a doctor's prescription, so you need to speak to your doctor.

How do you take Bactrim?

Carefully follow all instructions on your prescription label and check the medication guide that comes with Bactrim for more information.

Drink plenty of fluids if you're taking antibiotics, as they can cause diarrhea. If you have either watery or bloody diarrhea, tell your doctor.

Continue taking Bactrim even if your symptoms are rapidly improving. If you skip doses, your body and the bacteria may develop resistance to the medication.

Store Bactrim in a cool, dark place away from moisture, heat, and light. Do not refrigerate it.

Take Bactrim as prescribed by your health professional. Don't use Bactrim for any other complaints unless your doctor tells you to. Do not give Bactrim to anyone else, even if they appear to be suffering from the same illness as you.

The dosage of Bactrim will depend on the following factors;

  • The condition you’re treating

  • If you have impaired renal function

  • If it’s for prophylaxis (prevention)

Dosage forms and strengths

Bactrim: (Trimethoprim/sulfamethoxazole)

Oral suspension

  • (40mg/200mg)/5ml


  • 80mg/400mg

  • 160mg/800mg

Seeing results

Most people will start feeling relief after a few days of treatment for more common issues such as urinary tract and ear infections. However, it may take two to three weeks to notice improvement for more complicated conditions like pneumonia.

Potential side effects of Bactrim

You're unlikely to have any side effects from Bactrim. Some people experience itching or a skin rash when taking Bactrim, but this is usually not severe and goes away after you stop taking it.

Common side effects

Bactrim can cause common side effects:

  • Feeling or being sick

  • Itching or a mild rash

  • Headaches

  • Diarrhea

If these side effects bother you, talk to your doctor or pharmacist.

Serious side effects

Contact a healthcare provider straight away if you have:

  • Diarrhea that contains blood or mucus

  • Severe diarrhea that lasts longer than four days

  • If you have any of the following, as they could be signs of a problem in your blood: 

    • Bruising or bleeding you can't explain

    • A sore throat

    • Mouth ulcers

    • Fever

    • Feeling tired or generally unwell

Long-term use of Bactrim

While taking antibiotics long-term may have some risks, Bactrim is usually only taken for long periods if you have a weak immune system that can't fight off specific bacteria. In this case, because the bacteria can cause life-threatening infections, the benefits of taking Bactrim outweigh the risks.

Missed doses

If you miss a dosage of this medication, take it as soon as you remember. However, if it's nearly time for your next dose, skip the missed one and return to your regular dosing schedule. Don't double up on doses.


Overdose symptoms may include: 

  • Loss of appetite

  • Vomiting (sometimes bloody)

  • Fever

  • Yellowing of your skin or eyes (jaundice)

  • Confusion

  • Loss of consciousness

Seek immediate medical attention or call United States Poison Control at 1-800-222-1222.

What to discuss with your doctor before taking Bactrim

  • Discuss allergies you may have to:

    • Trimethoprim/sulfamethoxazole (Bactrim)

    • Sulfa

    • Diuretics (water pills)

    • Oral diabetes medications

    • Any other drugs 

  • Tell your doctor and pharmacist if:

    • You’re taking any prescription and nonprescription drugs or supplements, especially phenytoin (Dilantin)

    • You have anemia or liver or kidney disease

    • You're pregnant, planning to get pregnant, or breastfeeding. Call your doctor immediately if you become pregnant when taking trimethoprim (Bactrim).

Stopping Bactrim

Don't stop taking Bactrim unless you've checked with your doctor first. Don't let yourself run out of medication over the weekend or on holiday.

Bactrim and pregnancy

You can take Bactrim during pregnancy, but doctors don’t recommend it in the first 12 weeks. This is because it may affect your levels of folic acid, which is essential in the early stages of your baby's development.

Bactrim is sometimes the only suitable antibiotic for treating an infection. The benefits may outweigh the risks, but you should talk to your doctor about your pregnancy first.

If you need sulfamethoxazole/trimethoprim while trying to conceive or during the first 12 weeks of pregnancy, your doctor might recommend a high dosage of folic acid (5mg per day).

There's no need to take high doses of folic acid after 12 weeks, and Bactrim won't harm you if you keep taking it.

Interactions with other drugs

Severe interactions

Usually, doctors don’t prescribe these medications together. Ask your healthcare professional for more information.

  • Antimicrobials/live typhoid vaccine

  • Selected photosensitizers/tretinoin-mequinol

  • Sulfonamides/methenamine

  • Trimethoprim/dofetilide

Serious interactions

These medications may interact with each other and cause very harmful side effects.

  • Selected photosensitizers/aminolevulinic acid

  • Trimethoprim/digoxin

  • Trimethoprim/aldosterone receptor antagonists

  • Trimethoprim/potassium supplements

  • Trimethoprim/ACE inhibitors

  • Agents that affect radioactive iodide/iodide

  • Selected photosensitizers/porfimer

  • Selected antibiotics/live cholera vaccine

  • Sulfonamides

Moderate interactions

These medications may cause issues if taken together.

  • Sulfonamides/paba

  • Trimethoprim/metformin

  • Trimethoprim-sulfamethoxazole/leucovorin

  • Trimethoprim/pioglitazone

Allergy information

Tell your doctor if you have had a previous reaction to this medicine or others. Tell them if you have any other allergies, such as foods, dyes, preservatives, or animals. Read the labels or package ingredients carefully.

Clinical trial history

A study¹ of 30 senior women with recurrent urinary tract infections found that 12 out of 17 patients treated with Bactrim for 12 months had continuously sterile urine compared to just one out of 13 patients treated with nitrofurantoin.

A slight but statistically significant increase occurred in the serum creatinine level during treatment for 12 months with sulfamethoxazole plus trimethoprim, but this probably does not reflect any deterioration in kidney function.

Tips and advice for taking Bactrim

  • Finish all the antibiotics in your prescription, even if you start feeling better. Stopping Bactrim too soon may cause your condition to come back.

  • You can take Bactrim before or after meals. Some people experience side effects such as nausea and vomiting when taking this medication. If you experience these side effects, take Bactrim with meals to reduce them.

  • If you experience diarrhea after taking Bactrim, take probiotic supplements to alleviate this side effect. Continue taking the supplement for a few more days after finishing your last dose of Bactrim.

  • Bactrim (sulfamethoxazole/trimethoprim) makes your skin very sensitive to the sun, so you may burn quickly. This is true even when it's cool outside and cloudy. Avoid sunlight exposure, wear a wide-brimmed hat, and use sunscreen with an SPF of at least 30 while taking Bactrim.

  • Take Bactrim with 8oz of water and drink plenty of fluids while taking it. Bactrim passes through the kidneys, and insufficient water can cause kidney stones. Avoid drinking too many beverages that dehydrate you, such as coffee and alcoholic drinks.

  • Bactrim may interact with certain medications. Tell your pharmacist about all medicines you take, including over-the-counter drugs, vitamins, and herbal remedies.

  • Bactrim (sulfamethoxazole/trimethoprim) may cause allergic reactions. Sulfonamide antibiotics belong to a class of medications called sulfa drugs. Seek medical care immediately if you experience any signs of an allergic reaction while using Bactrim, such as a rash, hives, or swelling of the face. Tell your providers and pharmacy if you develop an allergy to Bactrim.

Curious about clinical trials?

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Here at HealthMatch, we’ve done our best to ensure that the information provided in this article is helpful, up to date, and, most importantly, accurate.

However, we can’t replace the one-to-one advice of a qualified medical practitioner or outline all of the possible risks associated with this particular drug and your circumstances.

It is therefore important for you to note that the information contained in this article does not constitute professional medical or healthcare advice, diagnosis or recommendation of treatment and is not intended to, nor should be used to, replace professional medical advice. This article may not always be up to date and is not exhaustive of all of the risks and considerations relevant to this particular drug. In no circumstances should this article be relied upon without independent consideration and confirmation by a qualified medical practitioner.

Your doctor will be able to explain all possible uses, dosages, precautions, interactions with other drugs, and other potential adverse effects, and you should always talk to them about any kind of medication you are taking, thinking about taking or wanting to stop taking.

Curious about clinical trials?

Access the latest treatments and medications. unavailable elsewhere - entirely free of charge. We make it easy to take part.