Bactrim (sulfamethoxazole/trimethoprim) is a brand-name drug that’s prescribed for certain infections. Bactrim comes as a tablet. The dosage can vary depending on the condition being treated.
Bactrim is approved by the Food and Drug Administration (FDA) to:
- treat urinary tract infections (UTI) in adults and children ages 2 months and older
- treat acute otitis media (ear infections) in children ages 2 months and older
- treat acute exacerbations of chronic bronchitis in adults
- treat shigellosis (a type of dysentery infection) in adults and children ages 2 months and older
- treat pneumocystis pneumonia (PCP) in adults and children ages 2 months and older
- help prevent PCP in adults and children ages 2 months and older with a weakened immune system
- treat traveler’s diarrhea in adults
Bactrim belongs to a drug class called sulfonamides. Bactrim is available in a generic version.
Keep reading for specific information about the dosage of Bactrim, including its strengths and how to take the medication.
Note: This article describes typical dosages for Bactrim provided by the drug’s manufacturer. When taking Bactrim, always follow the dosage prescribed by your doctor. However, your doctor will prescribe the Bactrim dosage that’s right for you.
The information below describes Bactrim’s typical dosages and other details about the drug.
Bactrim forms
Bactrim is available as oral tablets.
Bactrim strengths
Bactrim comes in two strengths:
- Bactrim: 400 milligrams (mg) sulfamethoxazole/80 mg trimethoprim
- Bactrim DS (“double strength”): 800 mg sulfamethoxazole/160 mg trimethoprim
Typical dosages in adults
The following information describes dosages that are commonly prescribed or recommended in adults. However, be sure to take the dosage your doctor prescribes for you. Your doctor will determine the best dosage to fit your needs.
Dosage for urinary tract infections (UTIs)
Doctors may prescribe Bactrim to treat urinary UTIs.*
If your doctor prescribes Bactrim for your mild to moderate UTI, your dose will likely be one 800 mg/160 mg tablet or two 400 mg/80 mg tablets. Typically, you’ll take this every 12 hours for 10–14 days. This is the typical recommended dosage of Bactrim for people with UTIs.
For more information about your specific dosage, talk with your doctor.
*Doctors may prescribe the intravenous (IV) form of this medication for severe UTIs. However, the brand-name Bactrim intravenous injection is no longer available in the United States, but generic versions are available. This article focuses on Bactrim and Bactrim DS, which only come as oral tablets. For more information about generic sulfamethoxazole/trimethoprim injection, talk with your doctor or pharmacist.
Dosage for chronic bronchitis
Bactrim is approved to help treat chronic bronchitis. In this case, the recommended dosage of Bactrim is one 800 mg/160 mg tablet or two 400 mg/80 mg tablets. You’ll take this dose every 12 hours for 14 days.
Talk with your doctor if you have questions about your dosage.
Dosage for shigellosis
Bactrim is also approved to help treat shigellosis. In this case, the recommended dose of Bactrim is one 800 mg/160 mg tablet or two 400 mg/80 mg tablets. Typically, you’ll take this dose every 12 hours for 5 days.
Talk with your doctor if you have questions about your dosage.
Dosage for pneumocystis pneumonia
Bactrim is also approved to treat pneumocystis pneumonia (PCP). Your dose of Bactrim will be based on your body weight in kilograms (kg). One kg equals about 2.2 pounds (lb). The daily dose will likely be 75–100 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 15–20 mg/kg if based on the trimethoprim component of Bactrim. Your total daily dose will be divided and given every 6–12 hours for 14–21 days.
The table below summarizes dosages of Bactrim when prescribed for PCP:
| Weight (lb) | Weight (kg) | Dose (every 6 hours) |
|---|---|---|
| 35 | 16 | one 400 mg/80 mg tablet |
| 53 | 24 | one and a half 400 mg/80 mg tablets |
| 70 | 32 | two 400 mg/80 mg tablets or one 800 mg/160 mg DS tablet |
| 88 | 40 | two and a half 400 mg/80 mg tablets |
| 106 | 48 | three 400 mg/80 mg tablets or one and a half 800 mg/160 mg DS tablet |
| 141 | 64 | four 400 mg/80 mg tablets or two 800 mg/160 mg DS tablets |
| 176 | 80 | five 400 mg/80 mg tablets or two and a half 800 mg/160 mg DS tablets |
Bactrim is also approved to help prevent PCP. In this case, the recommended dose of Bactrim is one 800 mg/160 mg tablet daily.
Talk with your doctor if you have questions about your dosage.
Dosage for traveler’s diarrhea
Bactrim is also approved to treat traveler’s diarrhea. In this case, the recommended dose is one 800 mg/160 mg tablet or two 400 mg/80 mg tablets. Typically, you’ll take this every 12 hours for 5 days.
Talk with your doctor if you have questions about your dosage.
Children’s dosage
The following sections describe Bactrim dosages for treating different conditions in children.
Dosage for urinary tract infections (UTIs)
Your child’s doctor may prescribe Bactrim to treat UTIs in children* ages 2 months and older.
The recommended daily dose is 40 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 8 mg/kg if based on the trimethoprim component of Bactrim. Your child’s total daily dose will be divided and given every 12 hours for 10 days. This dose comes in an oral tablet and is based on the child’s body weight in kilograms. One kg equals about 2.2 lb. The dosage per kilogram is measured in milligrams per kilogram (mg/kg) and is calculated by your child’s doctor.
The table below lists the typical oral dosing for children taking Bactrim:
| Weight (lb) | Weight (kg) | Dose (every 12 hours) |
|---|---|---|
| 44 | 20 | one 400 mg/80 mg tablet |
| 66 | 30 | one and a half 400 mg/80 mg tablets |
| 88 | 40 | two 400 mg/80 mg tablets or one 800 mg/160 mg DS tablet |
* Doctors may prescribe an oral suspension (liquid) form of this medication for adults and children who cannot swallow oral tablets. However, the brand-name Bactrim suspension is no longer available, but generic versions are available. This article focuses on Bactrim and Bactrim DS, which only come as oral tablets. For more information about generic sulfamethoxazole/trimethoprim oral suspension, talk with your doctor or pharmacist.
Talk with your child’s doctor if you have questions about their dosage.
Dosage for acute otitis media
Bactrim is also approved to treat acute otitis media (ear infections) in children ages 2 months and older. The recommended daily dose is 40 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 8 mg/kg if based on the trimethoprim component of Bactrim. Your total daily dose will be divided and given every 12 hours for 10 days.
The table below lists the typical oral dosing for children taking Bactrim:
| Weight (lb) | Weight (kg) | Dose (every 12 hours) |
|---|---|---|
| 22 | 10 | — |
| 44 | 20 | one 400 mg/80 mg tablet |
| 66 | 30 | one and a half 400 mg/80 mg tablets |
| 88 | 40 | two 400 mg/80 mg tablets or one 800 mg/160 mg DS tablet |
Talk with your child’s doctor if you have questions about their dosage.
Dosage for shigellosis
Bactrim is also approved to help treat shigellosis in children ages 2 months and older. The recommended daily dose is 40 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 8 mg/kg if based on the trimethoprim component of Bactrim. Your total daily dose will be divided and given every 12 hours for 5 days.
The table below lists the typical oral dosing for children taking Bactrim:
| Weight (lb) | Weight (kg) | Dose (every 12 hours) |
|---|---|---|
| 22 | 10 | — |
| 44 | 20 | one 400 mg/80 mg tablet |
| 66 | 30 | one and a half 400 mg/80 mg tablets |
| 88 | 40 | two 400 mg/80 mg tablets or one 800 mg/160 mg DS tablet |
Talk with your child’s doctor if you have questions about their dosage.
Dosage for pneumocystis pneumonia
Bactrim is also approved to treat PCP in children 2 months and older. Your child’s dose of Bactrim will be based on their body weight in kilograms. One kg equals about 2.2 lb. Their daily dose will likely be 75–100 mg/kg if calculated based on the sulfamethoxazole component of Bactrim and 15–20 mg/kg if based on the trimethoprim component of Bactrim. Your child’s total daily dose will be divided and given every 6–12 hours for 14–21 days.
The table below summarizes dosages of Bactrim when prescribed for PCP:
| Weight (lb) | Weight (kg) | Dose (every 6 hours) |
|---|---|---|
| 35 | 16 | one 400 mg/80 mg tablet |
| 53 | 24 | one and a half 400 mg/80 mg tablets |
| 70 | 32 | two 400 mg/80 mg tablets or one 800 mg/160 mg DS tablet |
| 88 | 40 | two and a half 400 mg/80 mg tablets |
| 106 | 48 | three 400 mg/80 mg tablets or one and a half 800 mg/160 mg DS tablet |
| 141 | 64 | four 400 mg/80 mg tablets or two 800 mg/160 mg DS tablets |
| 176 | 80 | five 400 mg/80 mg tablets or two and a half 800 mg/160 mg DS tablets |
Bactrim is also approved to help prevent PCP in children ages 2 months and older. In this case, the dose is calculated based on your child’s body surface area. The recommended dose is 750 milligrams per square meter per day (mg/m2/day) if calculated based on the sulfamethoxazole component of Bactrim and 150 mg/m2/day if calculated based on the trimethoprim component. The total dose is then equally divided to be given twice per day for 3 consecutive days per week. The maximum daily dose is 1,600 mg sulfamethoxazole and 320 mg trimethoprim.
The following table outlines the dosage for prevention of PCP in children:
| Body surface area (m2) | Dose (every 12 hours) |
|---|---|
| 0.53 | one-half 400 mg/80 mg tablet |
| 1.06 | one 400 mg/80 mg tablet |
Talk with your child’s doctor if you have questions about their dosage.
Short-term treatment
Bactrim is taken short term for various infections. Doctors prescribe Bactrim for 3–14 days, with the duration depending on the condition being treated and its severity. When prescribed to help prevent PCP, it’s taken long term.
Talk with your doctor if you have questions about how long you can expect to take Bactrim.
The Bactrim dosage your doctor prescribes will depend on several factors. These include:
- your body weight
- the type and severity of the condition you’re taking Bactrim to treat
- your kidney function
Dosage adjustments
Your doctor may need to adjust your dosage based on you or your child’s weight or the type or severity of the condition being treated.
Your doctor may also need to adjust your dosage if you have kidney disease.
Bactrim comes as an oral tablet.
It may be helpful to take Bactrim around the same time of day. This helps maintain a steady level of the drug in your body so Bactrim can work effectively.
If you have trouble swallowing tablets, see this article for tips on how to take this form of medication. You can also talk with your doctor or pharmacist.
If you have questions about how to use Bactrim, talk with your doctor or pharmacist.
ACCESSIBLE DRUG LABELS AND CONTAINERSSome pharmacies offer labels with large print, braille, or a code you scan with a smartphone to convert text to speech. If your local pharmacy doesn’t have these options, your doctor or pharmacist might be able to recommend a pharmacy that does.
If you’re having trouble opening medication bottles, ask your pharmacist about putting Bactrim in an easy-open container. They also may recommend tools that can make it easier to open bottles.
If you miss a dose of Bactrim, take it as soon as you remember. If it’s almost time for your next dose, skip the missed dose and take your next scheduled dose. Do not take two doses to make up for the missed one. If you’re not sure whether you should take a missed dose or skip it, talk with your doctor or pharmacist.
To help make sure that you don’t miss a dose, try using a medication reminder. This can include setting an alarm or putting a note where you’ll see it, such as on your bathroom mirror or bedside table. You could also download a reminder app on your phone.
It’s important that you do not take more Bactrim than your doctor prescribes. For some medications, taking more than the recommended amount may lead to harmful effects or overdose.
Effects of an overdose
Overdose effects of Bactrim can include:
- anorexia
- colic
- nausea
- vomiting
- dizziness
- headache
- drowsiness
- unconsciousness
- fever
- blood or crystals in urine
- blood disorders
- jaundice
- depression
- confusion
- bone marrow depression
If you take more than the recommended amount of Bactrim
Call your doctor right away if you believe you’ve taken too much Bactrim. Another option is to call America’s Poison Centers at 800-222-1222 or use its online tool. If you have severe symptoms, immediately call 911 or your local emergency number, or go to the nearest emergency room.
Below are some frequently asked questions about Bactrim.
Can Bactrim be used for skin infections? If so, what’s the dosage?
No, Bactrim is not used for skin infections. Bactrim is a brand-name drug that’s prescribed for treating or preventing other types of infections.
Because Bactrim isn’t used for skin infections, the drug’s manufacturer doesn’t provide recommended dosages for this use.
However, penicillin is a drug that’s approved for skin infections. If you have questions about managing your skin infection, talk with your doctor. They can recommend treatment options for skin infections.
How long does it take for Bactrim to start working?
Bactrim starts to work after your first dose. Because of how the drug works, you likely won’t feel the drug working in your body. But your doctor will monitor you during treatment to check whether the drug is working to treat your condition. Bactrim is a brand-name drug that’s prescribed for treating or preventing infections.
Talk with your doctor if you have questions about what to expect with Bactrim treatment.
The dosages in this article are typical dosages provided by the drug’s manufacturer. If your doctor recommends Bactrim for you, they will prescribe the dosage that’s right for you. Always follow the dosage that your doctor prescribes.
As with any drug, never change your dosage of Bactrim without your doctor’s recommendation. If you have questions about the dosage of Bactrim that’s best for you, talk with your doctor.
Besides learning about dosage, you may want other information about Bactrim. These additional articles might be helpful:
- More about Bactrim: For information about other aspects of Bactrim, refer to this article.
- Side effects: To learn about side effects of Bactrim, see the Bactrim prescribing information.
Disclaimer: Medical News Today 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.