Side Effects of Sulfamethoxazole/Trimethoprim

Allergic skin reactions, loss of appetite, and nausea are the most common side effects of sulfamethoxazole/trimethoprim. Even though a skin rash due to this medication is quite common, it should always be reported to your healthcare provider. Certain sulfamethoxazole/trimethoprim side effects are potentially serious and require immediate medical attention, including watery or blood diarrhea, decreased urination, and muscle weakness.

An Introduction to Side Effects of Sulfamethoxazole/Trimethoprim

As with any medicine, side effects are possible with sulfamethoxazole/trimethoprim (Bactrim®, Bactrim® DS, Septra®, Septra® DS, Sulfatrim®). However, not everyone who takes the medication will experience side effects. In fact, most people tolerate it quite well. If side effects do occur, in most cases, they are minor and either require no treatment or are easily treated by you or your healthcare provider.
(This article covers many, but not all, of the possible side effects with sulfamethoxazole/trimethoprim. Your healthcare provider can discuss a more complete list of sulfamethoxazole/trimethoprim side effects with you.)

Side Effects to Report

There are a number of possible side effects with sulfamethoxazole/trimethoprim that you should report immediately to your healthcare provider, including, but not limited to:
  • Signs of high potassium levels (hyperkalemia), including:


    • Abnormal sensations, such as burning, tingling, or pricking
    • Muscle weakness
    • Low heart rate (bradycardia)


  • Seizures
  • Watery or bloody diarrhea (which may occur long after you stop taking sulfamethoxazole/trimethoprim)
  • Signs of anemia, such as fatigue and pale skin
  • Signs of low white blood cells, such as frequent infections
  • Signs of low blood platelets, such as easy bruising or bleeding
  • Signs of kidney damage, such as very decreased urination
  • Signs of liver damage, such as yellow eyes or skin (jaundice), upper right abdominal pain, light-colored stools, or dark urine
  • Low blood sugar (hypoglycemia)
  • A cough or shortness of breath
  • Hallucinations
  • Depression
  • Signs of an allergic reaction, such as:


    • An unexplained rash
    • Hives
    • Itching
    • Unexplained swelling (especially of the lips, mouth, or throat)
    • Wheezing or other breathing problems.


A Dose of Reassurance for Parents of Picky Eaters

Sulfamethoxazole Trimethoprim Information

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