Bactrim (sulfamethoxazole/trimethoprim) is a prescription oral tablet used to treat and prevent certain types of bacterial infections. Your dosage will depend on the condition you’re taking the drug for and the tablet strength you’re prescribed.

The active ingredients in Bactrim are sulfamethoxazole and trimethoprim. Bactrim belongs to a group of drugs called sulfonamide antibiotics.

This article describes the dosages of Bactrim, as well as its strengths and how to take it. To learn more about Bactrim, see this in-depth article.

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.

This section describes the usual dosages of Bactrim. Keep reading to learn more.

What’s Bactrim’s form?

Bactrim is available as an oral tablet.

What strengths does Bactrim come in?

Bactrim comes in two strengths:

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

What are the usual dosages of Bactrim for adults?

The information below describes dosages that are commonly used or recommended. The dosage you’re prescribed and the length of your treatment depend on the type of infection you have.

Be sure to take the dosage your doctor prescribes for you. They’ll determine the best dosage to fit your needs. In some cases, doctors may adjust your dosage from those shown below.

If you have questions about your Bactrim dosage, talk with your doctor or pharmacist.

Dosage for urinary tract infection

Bactrim oral tablets are used to treat mild to moderate urinary tract infections (UTIs). The dosage for a UTI in adults is either two Bactrim tablets taken twice per day (every 12 hours) or one Bactrim DS tablet taken twice per day (every 12 hours).

You’ll take Bactrim or Bactrim DS for 10 to 14 days as directed by your doctor.

Note: For severe UTIs, an intravenous form of Bactrim may be prescribed. However, it’s only available in a generic version. This article focuses on the brand-name drugs Bactrim and Bactrim DS, which only come as oral tablets. If you have questions about the generic sulfamethoxazole/trimethoprim injection, talk with your doctor.

Dosage for traveler’s diarrhea

The Bactrim dosage to treat traveler’s diarrhea in adults is either two Bactrim tablets taken twice per day (every 12 hours) for 5 days or one Bactrim DS tablet taken twice per day (every 12 hours) for 5 days.

Dosage for chronic bronchitis

The Bactrim dosage to treat acute exacerbations of chronic bronchitis in adults is either two Bactrim tablets taken twice per day (every 12 hours) for 14 days or one Bactrim DS tablet taken twice per day (every 12 hours) for 14 days.

Dosage for pneumocystis pneumonia

The Bactrim dosage to treat pneumocystis pneumonia in adults is based on your body weight in kilograms (kg). For reference, 1 kg equals about 2.2 pounds (lb).

The recommended daily dose is 75 to 100 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 15 to 20 mg/kg if based on the trimethoprim component.

You’ll take this dose in equally divided doses every 6 hours for 14 to 21 days. Your doctor will calculate your Bactrim dosage for you.

The Bactrim dosage to prevent pneumocystis pneumonia in all adults is one Bactrim DS tablet taken per day.

Dosage for shigellosis

The dosage of Bactrim to treat shigellosis in adults is either one Bactrim DS tablet taken twice per day (every 12 hours) for 5 days or two Bactrim tablets taken twice per day (every 12 hours) for 5 days.

What’s the dosage of Bactrim for children?

Bactrim is used to help treat or prevent certain infections in children ages 2 months and older. The dosage is based on the child’s body weight in kg or their body surface area. See below for details.

If you have questions about your child’s dosage of Bactrim, talk with their doctor.

Dosage for urinary tract infection

The dosage for treating UTIs in children ages 2 months and older is based on their body weight in kg. Your child’s doctor will calculate their dosage.

The recommended daily dose is 40 mg/kg of body weight if calculated based on the sulfamethoxazole component of Bactrim and 8 mg/kg if based on the trimethoprim component. The daily dosage is divided into two equal doses, which are taken every 12 hours for 10 days.

Dosage for acute middle ear infection

The dosage to treat acute middle ear infections in children ages 2 months and older is based on their body weight in kg. Your child’s doctor will calculate their dosage.

The recommended daily dose is 40 mg/kg of body weight if calculated based on the sulfamethoxazole component of Bactrim and 8 mg/kg if based on the trimethoprim component of Bactrim. The daily dosage is divided into two equal doses, which are taken every 12 hours for 10 days.

Dosage for shigellosis

The dosage to treat shigellosis (a type of dysentery infection) in children ages 2 months and older is based on their body weight in kg. Your child’s doctor will calculate their dosage.

The recommended daily dose is 40 mg/kg of body weight if calculated based on the sulfamethoxazole component of Bactrim and 8 mg/kg if based on the trimethoprim component. The daily dosage is divided into two equal doses, which is taken every 12 hours for 5 days.

Dosage for pneumocystis pneumonia

The Bactrim dosage to treat pneumocystis pneumonia in children ages 2 months and older is based on their body weight in kg.

The recommended daily dosage is 75 to 100 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 15 to 20 mg/kg if based on the trimethoprim component of Bactrim. They’ll take this dose in equally divided doses every 6 hours for 14 to 21 days. Your child’s doctor will calculate their Bactrim dosage.

Bactrim may also be prescribed to prevent pneumocystis pneumonia in children ages 2 months and older. Their dosage is calculated based on their body surface area, which is measured in square meters (sq m). Body surface area is calculated from your child’s height and weight.

The recommended daily dosage is 75 to 100 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 15 to 20 mg/kg if based on the trimethoprim component. They’ll take this dosage in equally divided doses twice per day for 3 consecutive days each week.

The total maximum daily dose is 1,600 mg of sulfamethoxazole and 320 mg of trimethoprim.

How to take Bactrim

Bactrim comes as an oral tablet. You can take Bactrim with or without food. Your doctor will typically recommend taking Bactrim every 12 hours. Try to take the medication around the same time every day to keep steady levels of the medication in your body.

If you have trouble swallowing pills, you can cut Bactrim tablets in half before swallowing. You can also view these helpful tips on swallowing pills.

If you have questions about how to take Bactrim, talk with your doctor or pharmacist.

Is Bactrim used long term?

In most cases, no. Bactrim is usually taken short term to treat certain types of bacterial infections. The drug is only used long term to help prevent pneumocystis pneumonia in certain people. Your and your doctor will discuss how long you’ll likely take Bactrim.

Below are answers to some commonly asked questions about Bactrim’s dosage.

Is there a 3-day dosage of Bactrim for UTIs?

No, you’ll typically take Bactrim for 10 to 14 days for a mild to moderate UTI.

If you’re interested in a shorter treatment course for a UTI, talk with your doctor about your options. For example, a 3-day dosing regimen of the antibiotic ciprofloxacin (Cipro) may be prescribed in certain situations to treat a mild UTI.

Your doctor will determine the best treatment option and dosage for you. If you have questions about your Bactrim dosage, talk with your doctor or pharmacist.

Note: If you’re prescribed Bactrim to treat a UTI, it’s important to complete the full treatment course to prevent the infection from coming back.

Is Bactrim used for skin infections? If so, what’s the dosage?

No. Although Bactrim is used for certain infections, it’s not used to treat skin infections.

If you have questions about treatment options for a skin infection, talk with your doctor. They can recommend the best treatment option and dosage for you.

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:

  • fever
  • headache
  • drowsiness
  • dizziness
  • loss of consciousness
  • nausea and vomiting
  • blood in your urine
  • confusion
  • jaundice (yellowing of your skin or the whites of your eyes)

What to do in case you take too much Bactrim

Call your doctor right away 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.

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.