Bactrim for UTIs: Uses, Dosage & Side Effects

Written by: Jessica Guht
Jessica Guht
Categorized as UTI, Antibiotics
Jessica Guht
Categorized as UTI, Antibiotics

As one of the most common infections, UTIs have a range of antibiotics available as potential treatments. One of these options is Bactrim, which is also one of the most commonly prescribed treatments. However, increasing antibiotic resistance may no longer make this the best option.

Let’s discuss how Bactrim works, when it is a good choice, and when you may need an alternative antibiotic.

Table of Contents

What Is Bactrim?

Bactrim is the brand name for the combination medicine of sulfamethoxazole and trimethoprim. These antibiotics combine to treat various bacterial infections, including:

  • bronchitis
  • ear infections
  • intestinal infections
  • pneumonia
  • urinary tract infections (UTIs)
  • traveler’s diarrhea
  • meningitis

Bactrim belongs to the drug class sulfonamides, which stop bacteria growth by preventing bacteria from making a chemical (folic acid) they need to grow and survive.

Is Bactrim Used to Treat UTIs?

Bactrim is a common treatment for UTIs, and its treatment length may vary from 3 to 14 days, depending on the severity of the infection. This medication may also be used for children with UTIs if they are over two months old, with their dosage varying based on their weight.

For those with a UTI, Bactrim typically provides symptom relief within 3 days of the first dose.

However, Bactrim is only useful for UTIs due to bacterial infections. While this is the most common cause of a UTI, they may also result from fungi or a virus, and Bactrim is not a viable treatment option in these cases.

Is Bactrim Effective for UTIs?

While Bactrim is an effective treatment for many UTIs, it is not always as effective as it needs to be. This is the case with bacteria that are less sensitive to the medication, meaning the antibiotic cannot effectively stop the bacteria from growing. In these cases, an alternative medicine that is stronger is needed.

Other antibiotics that may be more effective against a UTI, and which may be used if Bactrim cannot clear the infection, include:


Bactrim is notably less effective against resistant bacteria, which is where Fosfomycin shines. This antibiotic kills the bacteria causing the UTI and prevents bacteria from sticking to the lining of the urinary tract.

Because of how powerful this antibiotic is, only one dose (typically 3g) is needed.


As an antibiotic that accumulates in the bladder, nitrofurantoin is specifically helpful for UTIs. It works by keeping bacteria from making the DNA and proteins they need in order to survive.

This antibiotic has a typical dosage of 100 mg twice a day for five days.

Amoxicillin/Potassium Clavulanate

Yet another option for resistant bacteria, the potassium clavulanate added to amoxicillin makes it more effective at destroying the bacteria’s cell wall. However, this one may still not be as powerful as Fosfomycin when it comes to resistant bacteria, and since amoxicillin is a penicillin, allergies can be common.

This medication is generally prescribed in a 500 mg tablet taken twice a day for 5-7 days.

Levofloxacin or Ciprofloxacin

Both of these antibiotics are slightly more effective than amoxicillin-potassium clavulanate. Still, this higher effectiveness also comes with a higher risk of serious side effects, including permanent effects to the nervous system, tendon rupture, or irregular heartbeats. Because of this, these antibiotics are generally reserved for more complicated UTIs.

Bactrim Dosage for a UTI

Bactrim tablets come in the following strengths: 400 mg/80 mg and 800 mg/160 mg. The first number is the amount of sulfamethoxazole, and the second is the amount of trimethoprim.

For UTIs in adults, the standard dose is 800 mg/160 mg, taken twice a day for 10 to 14 days.

For those taking Bactrim as a liquid suspension, shake the bottle well before measuring the dose.

If you miss a dose, taking it as soon as you remember is best. The only exception is if your next dose is within a few hours. In this case, skip the missed dose and begin taking it again at the next scheduled time. It’s important to never “double-up” on a dose you miss, as this may cause side effects and further complications.

Bactrim may be taken with or without food, although taking it with food may help anyone experiencing an upset stomach.

Bactrim Side Effects

Some of the common side effects of Bactrim include:

  • vomiting
  • nausea
  • rash
  • hives
  • diarrhea
  • loss of appetite
  • skin sensitivity to light or sun
  • changes to blood sugar

Of note, Bactrim can increase your sensitivity to light and the sun, meaning you are likely to sunburn more easily. As such, while taking this antibiotic, it’s recommended to avoid tanning beds and sunlight, and wear protective clothing and sunscreen when you have to go outdoors.

Bactrim may also cause some serious side effects, including:

  • infectious (C. difficile-related) diarrhea
  • serious allergic and skin reactions
  • kidney problems
  • low blood sodium levels
  • high potassium levels

Some symptoms indicative of the above conditions include:

  • urinating less than usual
  • swelling in the ankles, feet, or hands
  • nausea
  • headache
  • disorientation
  • seizures
  • hives
  • swelling of the tongue, lips, or throat
  • difficulty breathing
  • foul-smelling diarrhea
  • fever
  • muscle cramps

Contact your healthcare provider immediately if you notice any of the above symptoms.

Some people may also develop dangerous skin conditions, including toxic epidermal necrolysis (TEN) and Stevens-Johnson syndrome (SJS), although these complications are very rare.

Bactrim Drug Interactions

Bactrim can interact with many medications, including:

  • warfarin
  • leucovorin
  • ARDs
  • ACE inhibitors
  • diuretics

Be sure to let your doctor know about any medications and/or supplements that you are taking so that they can minimize drug interactions and potential side effects.

It is also not recommended for those with severe liver disease, anemia from folic acid deficiency, or unmonitored kidney disease to take Bactrim.

Additionally, this medication is not recommended for those who are pregnant or breastfeeding, as it may pass to the child/fetus and has been associated with birth defects. If you are taking Bactrim and become pregnant, inform your doctor so that they can switch you to another antibiotic.


What Should I Avoid While Using Bactrim?

It’s recommended to avoid alcohol while taking Bactrim, as it may cause a disulfiram-like reaction which can result in severe nausea and vomiting, headache, shortness of breath, and irregular heartbeat.

It’s also important to avoid taking new medications while on Bactrim unless you discuss it with your doctor first.

Because Bactrim can increase your sensitivity to light, it is also recommended to avoid tanning beds and sunlight. When you do have to go outside, wear protective clothing and sunscreen to protect your skin and prevent severe burns.

How Much Bactrim Can I Take for a UTI?

Your doctor will prescribe the dosage and frequency at which you should take Bactrim, but generally, the dosage is 800 mg/160 mg, taken twice a day for 10 to 14 days.

Taking more than prescribed will not make your infection go away any faster and will instead increase the risk of side effects.

How Quickly Does Bactrim Work for a UTI?

Bactrim will begin working to clear the infection within a few hours, but it may take a few days for you to notice an improvement in your symptoms. However, even if your symptoms go away, it is important to continue taking Bactrim for the complete prescribed course. Discontinuing it early increases the risk of bacteria remaining in the body, which could lead to a recurrent infection.

If your symptoms do not improve within a few days of taking Bactrim, reach out to your doctor; another antibiotic may be needed to treat your infection.

Is Bactrim a Strong Antibiotic for a UTI?

While Bactrim is an excellent general antibiotic for a UTI, it is not considered a strong antibiotic. This is because it is likely for bacteria to be less sensitive to it, meaning the antibiotic may not be able to clear the infection completely. In these cases, an alternative medication that is stronger is needed.

Some antibiotics which can be effective against resistant bacteria include Fosfomycin, levofloxacin, and ciprofloxacin.

However, stronger antibiotics may also have a higher risk of side effects, so your doctor will weigh the need for a stronger antibiotic with the potential risks.

What Kind of UTI Is Resistant to Bactrim?

Bactrim, as an antibiotic, is only effective against UTIs that result from bacterial infections. This means that it will not work as a treatment for UTIs that are due to fungal infections or viruses.

Furthermore, the New York City Department of Health reported in 2019 that one-third of uncomplicated UTIs due to E. coli (which is also the most common type of UTI) are resistant to Bactrim. This evidence shows that this medication, which is one of the most commonly prescribed antibiotics for UTIs, may not be effective in many cases.

Is Bactrim Better Than Amoxicillin for UTIs?

While both antibiotics can be used to treat UTIs, they have different methods by which they do so. Bactrim is a sulfonamide, which stops the growth of bacteria by stopping bacteria from making the chemical folic acid, which they need to grow and survive. In comparison, amoxicillin is a penicillin, which prevents bacteria from forming a cell wall, causing it to leech nutrients and die.

Bactrim may be better than amoxicillin when treating UTIs in some people. The biggest reason for this is if someone has a penicillin allergy, which can extend to amoxicillin. Since Bactrim is not a penicillin, it is safe to take for those with a penicillin allergy.

Key Takeaways

Bactrim is the brand name for the combination drug sulfamethoxazole/trimethoprim, an antibiotic that is effective against many bacterial infections, including UTIs. However, Bactrim cannot always clear the infection completely since it is not as effective against resistant bacteria. In these cases, a doctor will then go in with another antibiotic that is stronger.

If prescribed Bactrim, it is important to be aware of other drugs that it may interact with and signs of serious side effects. If any of these symptoms are present, seek immediate medical attention.

Bactrim may not be the best antibiotic choice to treat your UTI, especially if you have a history of antibiotic resistance. Be sure to discuss with your doctor if you have this history and the severity of your symptoms. With DrHouse, you can do all this from the comfort of your home, allowing you to receive the antibiotic you need to feel better without having to leave your house.


DrHouse articles are written by MDs, NPs, nutritionists and other healthcare professionals. The contents of the DrHouse site are for informational purposes only and are not a substitute for professional medical advice, diagnosis, or treatment. If you are experiencing high fever (>103F/39.4C), shortness of breath, difficulty breathing, chest pain, heart palpitations, abnormal bruising, abnormal bleeding, extreme fatigue, dizziness, new weakness or paralysis, difficulty with speech, confusion, extreme pain in any body part, or inability to remain hydrated or keep down fluids or feel you may have any other life-threatening condition, please go to the emergency department or call 911 immediately.

About DrHouse

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