8 Bactrim Side Effects and How to Manage Them

Bactrim (sulfamethoxazole / trimethoprim) comes as a single pill, but it’s actually a blend of two antibiotics. This duo increases Bactrim’s ability to destroy bacteria. It’s highly capable of treating gut, lung, and urinary tract infections, among others.

Bactrim can provide relief by clearing up your infection. But, as the old saying goes, there is no such thing as a free lunch. All medications have the potential to cause side effects;  Bactrim is no exception. Knowing Bactrim’s common side effects and how to manage them will help you treat your infection safely and effectively.

1. Nausea and vomiting

Gut-related issues such as nausea and vomiting are common Bactrim side effects. Generally, they are uncomfortable but mild enough to be managed at home. Taking each dose of Bactrim with a meal or snack is a good starting point. These at-home remedies can also help:

• Avoid spicy, fried, or high-fiber foods

• Try ginger or peppermint teas (or candies)

• Take an over-the-counter (OTC) medication such as bismuth subsalicylate (Pepto-Bismol) or Nauzene

If these steps don’t help, your healthcare provider may recommend a short-term prescription antinausea medication. Bactrim is typically prescribed only for a short period of time (less than 2 weeks), and the nausea should resolve after stopping it. But don’t stop taking this medication too early — this could make Bactrim less effective against future infections. 

If nausea persists despite your best efforts, talk to your healthcare provider about an alternative antibiotic.

2. Diarrhea

Bactrim can treat diarrhea caused by various bacterial infections. But Bactrim itself can cause diarrhea, too.

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If your stools are loose after starting Bactrim, try the following tips to treat diarrhea at home:

• Drink plenty of fluids, including electrolyte drinks, broth, or ginger tea.

• Take a probiotic, or eat probiotic-rich foods, to replenish good bacteria in your gut.

• Ask your provider if an OTC antidiarrheal medication is safe for you.

Diarrhea should go away after you finish taking Bactrim. However, sometimes a bad bacteria called Clostridioides difficile (“C. diff”) can spread in your gut after you take antibiotics. C. diff makes a toxin that irritates your gut, causing watery or bloody diarrhea, fever, or cramping. Thankfully, this is less common with Bactrim compared to many other antibiotics.  

Reach out to your healthcare team if you have bloody stools, fever, or if your diarrhea hasn’t improved with treatment.

3. Sun sensitivity

Certain medications make it more likely to get a sunburn after being in the sun. Both of Bactrim’s active ingredients, sulfamethoxazole and trimethoprim, have been linked to sun sensitivity.

But taking Bactrim doesn’t have to ruin your outdoor fun. Follow these tips to avoid a poor reaction:

• Avoid tanning beds and direct exposure to the sun.

• Wear sunscreen (SPF 30 or higher) while outdoors.

• Wear protective clothing such as long pants and long-sleeved shirts and a wide-brimmed hat.

• Be extra cautious with outdoor activities if you’re at a high elevation. This can increase the chance of a burn.

If you do get a mild sunburn, try using cold compresses or aloe vera gel. These can help soothe irritation. But if your sunburn is more severe and accompanied by fever, chills, or blistering, contact your healthcare provider.

4. Electrolyte changes

Bactrim can cause changes to the levels of potassium and sodium in your blood. Sodium and potassium are electrolytes — minerals in your body that help regulate hydration, nerve and muscle function, and heart rhythm.

Lower sodium or higher potassium levels typically happen within 3 to 10 days of starting Bactrim. They’re more likely with higher doses of the medication. A high potassium level is also more common if you have kidney issues or take other medications that increase potassium.

Your healthcare provider may monitor your sodium and potassium levels through regular blood draws while you’re taking Bactrim. Be sure to tell them if you’re experiencing symptoms of electrolyte changes, including:

• Nausea

• Headache

• Muscle weakness

• Sudden confusion

• Substantial fatigue

• Heart palpitations

• Shortness of breath

• Chest pain

What’s more, make sure that your healthcare provider has a list of all your health conditions and medications. Some Bactrim drug interactions — including with blood pressure medications, such as ACE inhibitors, and diuretics — can raise your risk for electrolyte changes. Your provider may choose a different antibiotic or suggest closer lab monitoring while you are taking Bactrim.

5. Low glucose levels

Bactrim can sometimes cause your blood glucose (sugar) to dip too low (hypoglycemia). This can happen to anyone, but it’s more problematic if you have diabetes. It can also enhance the effects of diabetes medications such as metformin and glipizide. 

Possible signs and symptoms of low glucose include:

• Sweating

• Shakiness

• Weakness

• Confusion

• Fast heartbeat

• Nervousness or mood change

If your glucose frequently drops too low while taking Bactrim, let your healthcare provider know. They may want you to monitor your glucose more frequently while taking Bactrim. They may also make adjustments to your antibiotic treatment plan.

6. Low platelet levels

Low platelet levels (thrombocytopenia) are possible with Bactrim. Platelets are blood cells that help your blood clot if you get a cut or scrape. Symptoms of low platelets include bruising and excessive bleeding.

In some cases, Bactrim can also cause immune thrombocytopenia. This occurs when Bactrim causes your immune system to destroy your platelets. It can be life-threatening if left untreated. Thankfully, this is reversible and treatable after stopping Bactrim. 

Bactrim interactions could also be at fault. Certain diuretics (water pills), such as hydrochlorothiazide, can decrease your platelets when you take them with Bactrim.

There isn’t much you can do to prevent low platelets. Your healthcare provider will likely catch any issues during regular blood draws, but you should still report any unusual bruising or bleeding to your healthcare team.

7. Skin rashes

Bactrim can cause skin reactions that are unrelated to sun exposure. Most skin reactions include a rash or raised bumps (hives). These bumps may be red, purple, or brown.

Rarely, Bactrim can cause more severe skin reactions. You may develop a blistering or peeling rash alongside other bodywide symptoms. Reactions such as Stevens-Johnson syndrome, toxic epidermal necrolysis, and drug reaction with eosinophilia and systemic symptoms have been reported in some cases.

Severe skin rashes from Bactrim can be life-threatening if they’re not promptly treated. If you have a rash that is peeling, blistering, or associated with fever, go to the nearest emergency room or call first responders.

To help prevent scary situations, report any new rashes to your healthcare provider — even if they’re mild. They could be an early sign of a reaction to Bactrim.

8. Sulfa allergies

The sulfamethoxazole portion of Bactrim is a “sulfa” medication. Sulfa medications contain a chemical known as sulfonamide. It can cause an allergic reaction in some people. This reaction typically happens within 1 to 2 weeks of starting Bactrim and resolves soon after stopping it. 

Skin rashes are just one possible symptom of a sulfa allergy. Other serious reactions may include trouble breathing or swelling of the face, lips, or tongue. Seek emergency medical care immediately for any of these symptoms.

In some scenarios, Bactrim may still be the best antibiotic for your infection — even if you have a sulfa allergy. This will depend on the severity of your allergy, which sulfa medication you have had an issue with, and other clinical factors.

When to talk to a healthcare provider about Bactrim side effects

You should feel empowered to reach out to your healthcare provider any time an issue arises while taking Bactrim. But some situations warrant extra attention. 

Contact your healthcare team right away if you experience any of the following symptoms:

• Uncontrolled nausea or vomiting

• Uncontrolled or bloody diarrhea

• Fever

• Rash or hives

• Extreme fatigue

• Confusion

• Muscle weakness

• Shakiness

• Unusual bruising or bleeding

These symptoms warrant emergency medical treatment:

• Blistering or peeling skin

• Chest pain

• Shortness of breath

• Heart palpitations

• Swelling of the face, lips, or tongue

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Written by:

Rachel Feaster, PharmD, BCOP, BCPS

Rachel Feaster has over 10 years of professional experience in ambulatory, inpatient, pharmacogenomics, and oncology care. She is board certified in oncology and pharmacotherapy.

Edited by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

Reviewed by:

Tegan Smedley, PharmD, APh

Tegan Smedley, PharmD, APh, has 10 years of experience as a pharmacist. She has worked in a variety of settings, including retail, hospital, and ambulatory care.

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