Bactrim (sulfamethoxazole/trimethoprim) is a prescription drug that’s used to treat or prevent certain infections in adults and some children. Bactrim comes as an oral tablet.

Bactrim basics

Bactrim belongs to a group of drugs called sulfonamide antibiotics. Bactrim is a brand-name medication that’s also available as the generic drug sulfamethoxazole/trimethoprim.

Note: Bactrim is available as a regular-strength oral tablet and a double-strength oral tablet (called Bactrim DS). This article will generally use the term “Bactrim” to refer to both strengths of the drug unless otherwise noted.

Like most drugs, Bactrim may cause mild to serious side effects. The lists below describe some of the more common side effects that Bactrim may cause. These lists don’t include all possible side effects.

Keep in mind that side effects of a drug can depend on:

  • your age
  • other health conditions you have
  • other medications you take

Your doctor or pharmacist can tell you more about the potential side effects of Bactrim. They can also suggest ways to help reduce side effects.

Mild side effects

Here’s a list of some of the mild side effects that Bactrim can cause. To learn about other mild side effects, talk with your doctor or pharmacist or read Bactrim’s prescribing information.

Mild side effects of Bactrim that have been reported include:

Mild side effects of many drugs may go away within a few days to a couple of weeks. But if they become bothersome, talk with your doctor or pharmacist.

Serious side effects

Serious side effects from Bactrim can occur, but they aren’t common. If you have serious side effects from Bactrim, call your doctor right away. But if you think you’re having a medical emergency, you should call 911 or your local emergency number.

Serious side effects of Bactrim that have been reported include:

Allergic reaction

For some people, Bactrim can cause an allergic reaction. This was a common side effect in studies of the drug.

In general, symptoms of an allergic reaction can be mild or serious. You can learn more about possible symptoms in this article.

What might help

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms and can determine whether you should keep taking Bactrim.

If you have symptoms of a severe allergic reaction, such as swelling or difficulty breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to Bactrim, they may have you switch to a different treatment.

Bactrim is used to treat the following types of infections:

Bactrim is also used to prevent pneumocystis pneumonia in adults and in children ages 2 months and older.

Infections occur when germs such as bacteria or fungus start to multiply in or on your body. In response, the immune system starts to fight the germs. This may cause symptoms such as fever or swelling. Other symptoms may occur that vary depending on the type of infection. For example, UTI symptoms commonly include frequent and painful urination.

Bactrim treats infections by killing germs, including certain types of bacteria and fungi. In some cases, your doctor may order tests to check the cause of your infection before prescribing Bactrim.

Note: Bactrim is not approved to treat long-term ear infections or to prevent ear infections.

Find answers to some commonly asked questions about Bactrim.

Is Bactrim similar to Macrobid, Cipro, or amoxicillin?

Bactrim is similar to these drugs in some ways but differs in others. Bactrim, nitrofurantoin (Macrobid), ciprofloxacin (Cipro), and amoxicillin are all used to treat infections. They all work by killing or eliminating germs that cause infection, such as bacteria.

However, these drugs have differences in:

  • the specific germs they’re effective against
  • some of the specific infections they’re used to treat
  • how long they’re taken

For example, ciprofloxacin is used to treat urinary tract infections (UTIs) in certain situations. Ciprofloxacin may be prescribed for 3 days for a UTI in some cases and 7 to 14 days in others. For treating UTI, Bactrim is taken for 10 to 14 days. Be sure to take the dosage your doctor prescribes.

Ask your doctor or pharmacist if you have questions about how Bactrim compares with other medications.

How long do Bactrim’s side effects last?

The common side effects of Bactrim usually go away within a few days. But some side effects, such as nausea, may continue for as long as you’re taking the medication.

In most cases, you’ll take Bactrim for 5 to 14 days, depending on your specific infection. Bactrim stays in your system for 2 to 3 days after your last dose. Any side effects you’re having should ease during this timeframe. However, this may take longer for people with kidney problems.

Bactrim may cause long-term side effects, but this isn’t common. One example is diarrhea caused by C. diff. In some cases, this side effect can occur for weeks to months after taking an antibiotic, such as Bactrim. Tell your doctor if you experience severe diarrhea and abdominal pain after taking Bactrim.

If you’re having side effects with Bactrim, talk with your doctor or pharmacist. They can recommend what to do.

Is Bactrim effective for treating cellulitis or STDs?

Possibly, but Bactrim is not approved to treat cellulitis or sexually transmitted infections (STIs).

However, doctors may prescribe Bactrim or Bactrim DS off-label for these uses. (With off-label use, doctors prescribe a drug for a purpose other than what it’s approved for.)

If you think you have an infection, it’s important to see your doctor. They can help identify the specific infection and recommend an effective way to treat it.

Your doctor will recommend the dosage of Bactrim that’s right for you. Below are commonly used dosages, but always take the dosage your doctor prescribes.

Form and strengths

Bactrim comes as an oral tablet. It’s available in two strengths:

  • Bactrim: 400 milligrams (mg) sulfamethoxazole/80 mg trimethoprim
  • Bactrim DS (double strength): 800 mg sulfamethoxazole/160 mg trimethoprim

Note: If you or your child are unable to swallow tablets, your doctor may instead prescribe the generic drug sulfamethoxazole/trimethoprim, which comes in an oral liquid form.

Recommended dosages

The recommended dosage of Bactrim depends on the type of infection being treated.

Dosage for urinary tract infection (UTI)

For treating UTI in adults, the recommended dose is either two tablets of Bactrim or one tablet of Bactrim DS. You’ll take this dose every 12 hours for 10 to 14 days, according to your doctor’s instructions.

For treating a UTI in children ages 2 months and older, the dose of Bactrim depends on your child’s body weight in kilograms (kg). Your child’s doctor will calculate the dose based on the two active ingredients in Bactrim.

The recommended dosage is 40 mg/kg of sulfamethoxazole and 8 mg/kg of trimethoprim per day. This will be divided into two doses per day (one dose taken every 12 hours) for 10 days.

Dosage for middle ear infection

For treating middle ear infection in children ages 2 months and older, the dose of Bactrim depends on your child’s body weight in kg. Your child’s doctor will calculate the dose based on the two active ingredients in Bactrim. The recommended dose is 40 mg/kg of sulfamethoxazole and 8 mg/kg of trimethoprim per day. This will be divided into two daily doses (one dose taken every 12 hours) for 10 days.

Dosage for shigellosis or traveler’s diarrhea

For treating shigellosis or traveler’s diarrhea in adults, the recommended dose is either two tablets of Bactrim or one tablet of Bactrim DS. You’ll take this dose every 12 hours for 5 days.

For treating shigellosis in children ages 2 months and older, the dose of Bactrim depends on your child’s body weight in kg. Your child’s doctor will calculate the dose based on the two active ingredients in Bactrim. The recommended dose is 40 mg/kg of sulfamethoxazole and 8 mg/kg of trimethoprim per day. This will be divided into two daily doses (one dose taken every 12 hours) for 5 days.

Dosage for exacerbation of chronic bronchitis

For treating an exacerbation (sudden worsening) of chronic bronchitis in adults, the recommended dose is either two tablets of Bactrim or one tablet of Bactrim DS. You’ll take this dose every 12 hours for 14 days.

Dosage for pneumocystis pneumonia

For treating pneumocystis pneumonia in adults and in children ages 2 months and older, the dose depends on body weight in kg. Your doctor will calculate your dose based on the two active ingredients in Bactrim. They’ll tell you how many tablets to take for each dose.

For this use, the recommended dose is 75 to 100 mg/kg of sulfamethoxazole and 15 to 20 mg/kg of trimethoprim. This will be divided into four doses per day (one dose taken every 6 hours) for 14 to 21 days.

To prevent pneumocystis pneumonia in adults, the recommended dose is one tablet of Bactrim DS taken once per day.

To prevent pneumocystis pneumonia in children ages 2 months and older, the dose depends on the child’s body surface area (BSA), which is measured in squared meters (m2). Your child’s doctor will calculate the BSA using their height and weight. Then, they’ll calculate the dose and tell you how many tablets to give for each dose.

For this purpose, Bactrim is typically taken twice per day for 3 consecutive days each week.

Questions about taking Bactrim

Below are some common questions about taking Bactrim.

  • Can Bactrim tablets be split? Yes, Bactrim tablets may be split in half. If you or your child have difficulty swallowing pills, you can also talk with your doctor. They may prescribe an oral liquid version of the generic drug sulfamethoxazole/trimethoprim instead. Talk with your doctor to learn more.
  • Should I take Bactrim with food? Bactrim can be taken with or without food. But you should take each dose with plenty of water. This may help prevent kidney stones, which is a possible side effect of the drug.
  • Is there a best time of day to take Bactrim? There isn’t a best time of day to take the drug, but Bactrim is usually taken every 12 hours. Taking the medication on a consistent schedule, as prescribed by your doctor, helps the drug work effectively.
  • What if I miss a dose of Bactrim? If you miss a dose of Bactrim, take it as soon as you remember. However, if it’s almost time for your next dose, skip the missed dose and take your next dose at its scheduled time. If you’re taking Bactrim to treat an infection, it’s important to finish all of the medication so the infection doesn’t come back. If you’re not sure what to do after missing a dose, talk with your doctor or pharmacist.
  • Will I need to use Bactrim long term? Bactrim is typically used short-term to treat certain bacterial infections. It’s only used long term if taken to help prevent a fungal infection called pneumocystis pneumonia in certain people.
  • How long does Bactrim take to work? Bactrim starts working just after you take your first dose. To treat an infection, Bactrim is usually taken for 5 to 14 days. It’s important to finish all of the medication so that the infection does not come back. Be sure to follow your doctor’s instructions.

Overdose

Do not take more Bactrim than your doctor prescribes, as this can lead to harmful effects.

Symptoms of overdose

Symptoms caused by an overdose can include:

  • digestive issues, such as nausea, vomiting, and low appetite
  • headache
  • drowsiness
  • dizziness
  • fever
  • confusion
  • blood in your urine
  • jaundice (yellowing of your skin or the whites of your eyes)
  • loss of consciousness

What to do in case you take too much Bactrim

Call your doctor if you think you’ve taken too much Bactrim. You can also call 800-222-1222 to reach America’s Poison Centers or use its online resource. But if you have severe symptoms, immediately call 911 or your local emergency number. Or go to the nearest emergency room.

Whether you have health insurance or not, cost may be a factor when you’re considering Bactrim. What you’ll pay for Bactrim may depend on several things, such as your treatment plan and the pharmacy you use.

Here are a few things to consider regarding cost:

  • Cost information and savings coupons: You can visit Optum Perks to get price estimates of what you’d pay for Bactrim when using coupons from the site. See the coupon options below. (Note: Optum Perks coupons cannot be used with any insurance copays or benefits.)
  • Generic form: Bactrim is available as the generic drug sulfamethoxazole/trimethoprim. Generics usually cost less than brand-name drugs. Talk with your doctor if you’d like to know whether sulfamethoxazole/trimethoprim is an option for you.

You can also check out this article to learn more about saving money on prescriptions.

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Retail price refers to the manufacturer’s published list price and is up to date as of 3/2023. Retail and discounted prices are U.S.-only and can vary based on region and pharmacy. We cannot guarantee that the discounted price listed here will exactly match the price at your pharmacy. Please contact your pharmacy for the exact price.

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Below is important information you should consider before taking Bactrim.

Interactions

Bactrim can interact with several other medications. It may also interact with certain lab tests.

Before taking Bactrim, talk with your doctor and pharmacist. Tell them about all prescription, over-the-counter, and other drugs you take. Also tell them about any vitamins, herbs, and supplements you use. Sharing this information can help you avoid potential interactions.

Drug interactions

Below are examples of medications that can interact with Bactrim. This list does not contain all drugs that may interact with Bactrim. If you have questions about drug interactions that may affect you, ask your doctor or pharmacist.

Drug group or drug nameDrug examples
diuretics (water pills)• hydrochlorothiazide (Microzide)
certain diabetes medications• glipizide (Glucotrol XL)
• pioglitazone (Actos)
angiotensin converting enzyme inhibitors • lisinopril (Zestril)
• ramipril (Altace)
tricyclic antidepressants• amitriptyline
• ortriptyline (Pamelor)
certain heart medications• dofetilide (Tikosyn)
• digoxin (Lanoxin)
warfarin (Jantoven)—
methotrexate (Trexall)—

Other interactions

  • Foods: Bactrim may be taken with or without food. Bactrim doesn’t have any interactions with food, so you don’t need to avoid any specific foods while taking Bactrim.
  • Lab tests: Bactrim may interfere with the results of certain lab tests. These include tests that measure creatinine and tests that measure the level of the drug methotrexate in your blood.

Bactrim and alcohol

Alcohol is not known to interact with Bactrim. Talk with your doctor to determine whether it’s safe for you to consume alcohol during Bactrim treatment.

Pregnancy and breastfeeding

Bactrim is not safe to take during pregnancy or while breastfeeding. If you’re pregnant or breastfeeding, or planning to become pregnant or to breastfeed, talk with your doctor before starting Bactrim treatment.

Birth control

If you can become pregnant, you should use birth control throughout your Bactrim treatment.

Warnings

Bactrim can sometimes cause harmful effects in people who have certain conditions. This is known as a drug-condition interaction. Other factors may also affect whether Bactrim is a good treatment option for you.

Talk with your doctor about your health history before taking Bactrim. Be sure to tell them if any of the following factors apply to you or your child:

Other drugs are available that are similar to Bactrim. If you’d like to explore an alternative to Bactrim, talk with your doctor. They can tell you about other medications that might work well for you.

The following drugs are similar to Bactrim:

  • sulfamethoxazole/trimethoprim (Sulfatrim Pediatric)
  • sulfamethoxazole/trimethoprim (Septra, Septra DS)

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.