Warnings


Bactrim Risks, Warnings, and Complications



Bactrim can cause serious health problems, especially in certain high-risk groups. Some of the potential risks of taking Bactrim are listed below.

  • ALLERGIC REACTIONS: An allergic reaction to a sulfa drug like Bactrim may lead to life-threatening airway swelling or anaphylaxis. Call 911 if this occurs. Get emergency medical help if you notice signs of severe allergies such as hives, rash; lip, tongue, or throat swelling; or difficulty breathing or swallowing

  • SEVERE SKIN REACTIONSRarely do people develop severe and potentially life-threatening skin reactions after taking Bactrim. These types of severe skin reactions include painful blisters all over the body. Get emergency medical help if you notice a skin rash, fever, skin sores, blisters, or mouth sores.

  • SEVERE DIARRHEA: Like other antibiotic medicines, Bactrim can cause diarrhea by eliminating good bacteria from the gut. Rarely, this diarrhea may be caused by a bacteria called Clostridioides difficile (C. diff), which is normally kept in check by healthy gut bacteria. C. diff diarrhea is a severe intestinal condition that is highly contagious and can quickly lead to life-threatening dehydration. The risk of C. diff diarrhea is higher in people taking multiple antibiotics or those on antibiotics for prolonged or repeated periods. Also, people over the age of 65, those who are hospitalized, those with weakened immune systems are at risk, and those who have had previous exposure to C. diff are at risk. Wash your hands with soap and water for 20 seconds frequently. If anyone in your household has been diagnosed with C. diff, others should try to use a different bathroom. Probiotic supplements while on Bactrim may help prevent C. diff diarrhea.

  • RHEUMATIC FEVER: Bactrim does not treat all bacterial infections, including scarlet fever and strep throat. Do not take Bactrim unless prescribed by a healthcare professional. Taking Bactrim for an infection it cannot treat can lead to worsening the infection. In some cases, it can lead to a condition called rheumatic fever, which is characterized by fever, muscle aches, and inflamed joints. Contact your doctor immediately if you have a fever, muscle aches, or unusual joint pain while on Bactrim.

  • LOW PLATELET COUNT: A rare but potentially life-threatening side effect of Bactrim is thrombocytopenia (low blood platelets). This problem usually corrects within a week of stopping Bactrim. Get emergency medical help if you experience fever, weakness, purple skin rash, bruises in the skin, unusual bleeding, or yellowing of the skin or eyes.

  • SERIOUS DRUG REACTION WITH LEUCOVORIN: People with HIV may be on a medication called leucovorin. This HIV medicine should never be taken with Bactrim as it can increase the risk of death. If you have HIV and are on leucovorin, tell your healthcare provider, especially if they prescribe a medication to treat pneumonia.

  • LOW BLOOD SUGAR: In a small number of people, taking Bactrim can lead to low blood sugar for a few days. This can happen even to people who don’t have diabetes. The risk of this occurring is higher in people with kidney or liver disease, poor nutrition, or taking high doses of Bactrim. Very low blood sugar is a potentially life-threatening problem. Symptoms can include nervousness, anxiety, shakiness, fast heartbeat, chills or sweating, confusion, sleepiness, low energy, severe weakness, excessive hunger, excessive thirst, or fruity breath odor.

  • BIRTH DEFECTS: Bactrim can cause harm to unborn babies and lead to birth defects. Tell your doctor if you are pregnant or could be pregnant before starting Bactrim. Stop taking Bactrim and call your doctor if you become pregnant while on this medication. 

Talk to your doctor or pharmacist about the risks of taking Bactrim. Your healthcare professional can prescribe other drugs to treat infections if you are at risk of complications. Call your doctor immediately if you experience any unusual symptoms while on Bactrim.


Precautions Before Starting Bactrim

Tell your doctor if you have previously had allergic reactions to Bactrim (sulfamethoxazole and trimethoprim), any other antibiotics, or any other drugs. Your pharmacy can give you a list of active and inactive ingredients in Bactrim tablets.

Give your doctor a complete medical history. Bactrim may not be right for people with certain medical conditions such as kidney disease, liver disease, megaloblastic anemia due to folic acid deficiency, and in pregnant women and nursing mothers.

Give your doctor a complete list of all your medications, including prescription and over-the-counter drugs, nutritional supplements, vitamins, and herbal products — this can help avoid serious drug interactions.

Tell your doctor if you are pregnant, plan to become pregnant, or are breastfeeding. Bactrim can harm unborn babies and cause birth defects. This medication can pass into breast milk and is not suitable for nursing mothers. Bactrim is not recommended for infants under the age of 2 months.

Precautions During Use of Bactrim

Keep all your healthcare appointments. Your doctor may want to do certain blood tests or urine tests to check if your infection has cleared and to ensure Bactrim is not causing any serious adverse reactions.

Bactrim can cause sun sensitivity. Be sure to avoid sun exposure as much as possible and wear protective clothing and sunscreen while on this medication.

Call your doctor or seek emergency medical help if you develop signs and symptoms of allergic reactions, severe skin reaction, low blood platelets (unusual bruising), low blood sugar, intestinal infections (severe diarrhea), high blood potassium (nausea, weakness, tingling, chest pain, irregular heartbeats), or low blood sodium (headache, weakness, confusion, memory problems) while on Bactrim.

Stop taking Bactrim and call your doctor if you become pregnant while taking Bactrim.

Bactrim Drug Interactions

Taking Bactrim with other medications can lead to reduced or elevated plasma concentrations and affect how the medicines work. Drug interactions can also increase the risk of serious adverse effects. Your physician may choose a different medication, change the dose or frequency of one or both medicines, and/or monitor you carefully for side effects if there are known interactions between Bactrim and your other drugs.

Taking Bactrim with the following medications is not recommended:

  • Leucovorin (used in HIV patients and as an antidote to the cancer medication methotrexate)
  • Methenamine (used to treat bladder infections)
  • Levomethadyl (used to manage opiate dependence)
  • Dofetilide (used to treat irregular heart rhythm)


There can also be drug interactions between Bactrim and medicines, such as:

  • Thiazide diuretics (water pills)
  • Warfarin (anticoagulant or blood thinner)
  • Phenytoin (an anti-seizure medication)
  • Methotrexate (a cancer medication)
  • Cyclosporine (a medication used to prevent transplant rejection)
  • Digoxin (a heart medication)
  • Indomethacin (an NSAID or nonsteroidal anti-inflammatory drug)
  • Pyrimethamine (a medication used for malaria prevention)
  • Tricyclic antidepressants
  • Certain oral diabetes medicines like pioglitazone, repaglinide, rosiglitazone, glipizide, glyburide, and metformin


This list may not describe all possible Bactrim interactions. Give your healthcare providers a complete list of your medications, including prescription drugs, over-the-counter medicines, dietary supplements, and herbal products. Also, tell your healthcare providers if you smoke, drink alcohol, or use illegal drugs because some of these substances can cause serious health complications or increase the risk of side effects when used with prescription medications.