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What Are the Most Common Sulfamethoxazole Side Effects?

By Bobbie Fredericks
Updated May 17, 2024
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The most common sulfamethoxazole side effects are mild stomach upsets, such as nausea and diarrhea. It can also cause increased sensitivity to the sun, so patients using this drug should take precautions when sun exposure is unavoidable. A hat, long sleeves, long pants, and sun block should all be used. There are also serious, rare reactions that may occur.

Sulfamethoxazole is usually combined with trimethoprim. They are both antibiotics in the sulfanomide family, which means their potential side effects are the same. This combination is used most often to treat bacterial urinary tract infections caused by E. coli. It can also be used for ear infections, and others as determined by a health care provider. Experts believe that both drugs have the potential to cause birth defects.

Nausea, diarrhea and anorexia are the most common sulfamethoxazole side effects. Also possible are vertigo, headache, and general abdominal pain. A fever or rash may also occur as minor side effects, but they could also signal a more serious condition, such as Stevens-Johnson Syndrome. Anaphylaxis, a serious and possibly life threatening allergic reaction, is possible with any drug, but particularly with antibiotics. Swelling of the tongue and wheezing signal a medical emergency. Patients who have ever had a reaction to a sulfanomide should not take sulfamethoxazole.

Stevens-Johnson Syndrome is probably the most severe of the sulfamethoxazole side effects. Symptoms of this disorder include fever, itching, skin lesions and sores in the mouth. Since the skin sheds from the body, patients with Stevens-Johnson Syndrome must be treated in a hospital, sometimes in a burn unit. There is a high risk of infection since there are large open wounds. In some cases there is permanent skin damage, blindness, or organ damage.

Other serious sulfamethoxazole side effects include hepatitis, bloody diarrhea, seizures, and an enlarged thyroid. Patients experiencing these side effects should seek immediate medical attention. Most of these side effects can be treated and do not cause lasting damage.

Certain sulfamethoxazole side effects are associated with long-term use of the drug. This drug may cause a decrease in blood platelets, which leads to excessive bruising or bleeding. Patients who are taking this drug should contact his or her doctor immediately if this side effect is noticed. This drug may also cause hemolytic anemia, which is caused by the premature destruction of red blood cells or megaloblastic anemia, which is caused by enlarged red blood cells. Some patients may need to have periodic blood tests to monitor their platelet levels.

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Discussion Comments

By anon1003643 — On Aug 06, 2020

Beware. I was prescribed and experienced side effects of headaches, chills, Night sweats. Got bloodwork done and my white blood cell count and platelets are low. Dr. prescribed a different antibiotic and I will not be filling that prescription. Instead, I scheduled an appointment with my naturopathic doctor. There just has to be alternative remedies.

By anon974657 — On Oct 20, 2014

After three doses, I questioned my health care doctor who prescribed this because I started experiencing rapid heart rate, throbbing throughout my entire body, chills, shaking, nausea and bloodshot eyes. I was assured over the phone it was not due to the antibiotic, but they suggested I go to the ER.

It was there my liver enzymes were noted as elevated and inflamed. I was sent home to rest but advised to continue the Bactrim. My husband returned me to the ER in a short amount of time due to worsening symptoms. they did more blood work and a spinal tap and admitted me. My internal medicine doctor said the adverse side effects were due to the Bactrim.

My symptoms extended to severe joint pain and swelling, headaches, increased elevation of liver enzymes (hepatic necrosis), continued bloodshot eyes and nausea.

It is now a month post prescribed Bactrim, and I still have severe joint and muscle pain, my bloodwork is still not normal, I have difficulty digesting meals, bloodshot eyes and headaches. I later found out I have several family members who almost died from adverse sulfa drug effects.

By fBoyle — On Dec 23, 2012

@feruze-- But there is no way for people to know if they're allergic until they take this drug and allergic reactions to sulfamethoxazole is more common than people realize. It's especially common in kids and I don't think this medicine should be prescribed to children or the elderly under any condition. Children and elderly have a much harder time recuperating from the side effects of this drug.

My friend's daughter had an allergic reaction to sulfamethoxazole and suffered from hives, swelling and difficulty breathing for months. I don't know how many times they ended up at the emergency room.

By bear78 — On Dec 22, 2012

@donasmrs-- It's good to be aware of the side effects but you shouldn't ignore the doctor's prescription because your infection will just get worse.

I took sulfamethoxazole for a week and the only side effects I had was fatigue and a headache on some days. It's not such a terrible drug and it cleared up my infection in one week.

Unless you are allergic to this group of antibiotics, I don't see why you shouldn't take it when it's been prescribed.

By donasmrs — On Dec 21, 2012
Oh my God! I have just been prescribed this medication for a urinary tract infection. I wanted to check potential side effects before I started the treatment and I'm glad I did. I don't think I've ever seen an antibiotic with so many serious side effects before. This must be a super strong drug. I don't think I will take it.
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