What are the Different Trimethoprim Side Effects?

Jillian O Keeffe

Trimethoprim is an antibiotic used in the treatment of urinary tract infections, traveler's diarrhea and pneumonia. The drug's most common side effects involve skin complaints, thought other trimethoprim side effects include allergic reactions, gastrointestinal symptoms, and an effect on blood cells and electrolyte levels. A rare side effect of taking trimethoprim is developing meningitis.

A sore throat from trimethoprim should be reported to a doctor.
A sore throat from trimethoprim should be reported to a doctor.

Skin rash and pruritis, a condition in which the skin becomes itchy, are among the most common of the trimethoprim side effects. Rash will occur in 2.9 percent to 6.7 percent of people taking the standard 10-day regimen of 100 milligrams (mg) or 200 mg. The rash can also be sensitive to sunlight. Taking higher doses of the drug increases the likelihood of the rash occurring. The rash occurs from one to two weeks after beginning the antibiotic therapy and is generally of mild to moderate severity.

Trimethoprim is sometimes prescribed to treat traveler's diarrhea.
Trimethoprim is sometimes prescribed to treat traveler's diarrhea.

The next most common adverse effects of trimethoprim include gastrointestinal symptoms such as nausea, vomiting and stomach upset. More severe gastrointestinal symptoms, which require medical attention, are severe diarrhea, bloody or tarry stools, stomach cramps, decreased urination, vaginal irritation and vaginal discharge. Other side effects that require medical attention include chest pain, fever, chills, sore throat, unusual bruising or bleeding, unusual tiredness or weakness, painful or stiff neck, unusually pale skin, joint or muscle pain, seizures, purple skin patches or yellowing of the skin or eyes.

Trimethroprim can be prescribed to treat urinary tract infections.
Trimethroprim can be prescribed to treat urinary tract infections.

Trimethoprim works by preventing folic acid synthesis in bacteria, because the bacteria must synthesize folic acid if it is to grow. The drug is, therefore, commonly used in conjunction with a sulfonamide drug, which blocks a different step in bacterial folic acid synthesis. Trimethoprim combination therapy is most commonly used for treating pneumonia.

Trimethoprim combination therapy is most commonly used to treat pneumonia, which can be diagnosed via chest X-ray.
Trimethoprim combination therapy is most commonly used to treat pneumonia, which can be diagnosed via chest X-ray.

Trimethoprim's inhibition of folic acid synthesis may also make patients who are deficient in folic acid more prone to a certain type of anemia, called megaloblastic anemia. The different trimethoprim side effects also include other blood disorders such as leukopenia and thrombocytopenia. The drug could cause jaundice in rare cases. Trimethoprim may also, in rare cases, cause aseptic meningitis, though developing meningitis as a side effect may be more common in patients with connective tissue diseases such as lupus.

Nausea is one possible side effect of taking trimethoprim.
Nausea is one possible side effect of taking trimethoprim.

The list of trimethoprim side effects includes severe allergic reaction to the drug. The signs of an allergic reaction to trimethoprim include hives; rash; itching; difficulty breathing; swelling of the face, neck and tongue; and tightness in the chest. The drug could also cause electrolyte imbalances such as low sodium levels or high potassium levels.

Itchiness may be a side effect of trimethoprim.
Itchiness may be a side effect of trimethoprim.

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Discussion Comments


I'm taking trimethoprim, but I'm getting worried because I'm finding it hard to breathe and I feel light headed. Is this normal?


I guess I'm one of the lucky ones. I only experienced mild stomach discomfort while on trimethoprim, and even this might have been caused by something else.

It just so happened that while I was taking the drug, I was under a ton of stress at work. This very well could have caused my stomach upset, so I can't be sure that trimethoprim was fully to blame.

I lost my appetite during those ten days, and I winded up losing a few pounds. I wasn't upset about this at all!

I didn't have bad cramps, but I did feel a little queasy. I simply did not want to eat. I didn't feel like I might vomit or anything, but I just didn't want any food, which is unusual for me.


I got a rash while on trimethoprim, but my doctor had already warned me this might happen. It was mildly itchy and not too apparent, so I decided to continue taking the course of antibiotics.

Where I messed up was when I went out into the sun. I had a few days off of work, and the weather was nice, so I decided to do a little yard work. Since it was early spring, I didn't think I would get sunburned, but I forgot that the medication and the rash made me far more sensitive to the sun than usual.

Within forty minutes, I had a full-fledged sunburn. This made me itch even more, and I literally wanted to peel my skin off. I applied aloe vera gel, which helped to soothe the itching and burning, and I had to get my doctor to prescribe me a different antibiotic, because the itching was just too intense at that point.


@orangey03 – I never got to find out if trimethoprim would cure my urinary tract infection or not, because I could not endure the side effects. Instead of simple nausea, I had to vomit several times. This was something I could not handle, and I have a feeling I was probably throwing up at least some of the medicine, so it wasn't doing me any good.

I decided to stop taking the drug and try a natural remedy I had heard about. I drank two eight-ounce glasses of cranberry juice each day, and I also took cranberry extract pills. I ate pineapple and yogurt, too, because are both known to combat the bacteria that causes a urinary tract infection.

After a week, I felt completely cured. I no longer have to rely on antibiotics like trimethoprim to relieve me from urinary tract infections.


I took trimethoprim to treat a urinary tract infection, and though I did suffer some side effects, I must say that the relief from the condition I was taking it for far outweighed the negatives. I was so tired of needing to urinate every thirty minutes and having bladder cramps that I would have endured even more bad effects than I experienced to get rid of the condition.

The drug did make me nauseous. I would have sudden waves of nausea while working at my desk, and I would feel as though I might vomit. However, I never actually did.

I found that sucking on peppermint candy helped control my nausea. Granted, I had to keep one in my mouth almost constantly to keep it at bay, but it helped so much that I did not mind.


Very seldom do I have any kind of allergic reaction to a medication, but I sure did when I was prescribed Timethorprim.

One winter I came down with a bad case of pneumonia and this is the first antibiotic I was given. Within 24 hours I was itching and had a rash that covered most of my body.

My doctor told me to immediately stop taking the Trimethoprim and gave me a different antibiotic. I had taken this antibiotic before I knew I hadn't had any problems with it.

As soon as I stopped taking the Trimethoprim, my itching stopped and the rash went away. I am thankful there are a lot of other antibiotics to choose from.

One of my friends has an allergic reaction to almost every kind of antibiotic she has taken. Trimethoprim is one of the rare antibiotics she can take and not have a reaction.

Everybody is made up differently, and we don't know how we are going to react to something until it gets in our system.


It seems like every time I travel someplace I end up getting diarrhea. At first I thought it had something to do with the food and water I was drinking, but my doctor told me I probably had traveler's diarrhea.

This is something I had not heard of before, but she was it was very common. She gave me a prescription for Trimethoprim to see if that would help with the symptoms when I went on my next trip.

It worked really great, and it was so nice to not have to worry about running to the bathroom all the time. I didn't notice any negative side effects with it at all.

My sister tried this too, but she ended up having a lot of nausea and GI problems. It seems like there are a lot of medications that have side effects that are similar to what you are trying to prevent.

In her case, she was trying to prevent getting diarrhea, yet one of the side effects of this medication is diarrhea. She ended up not using it because the side effects were worse than her symptoms.


@ddljohn - I have a history of urinary tract infections and have tried several medications for this through the years.

They don't like to keep giving me the same antibiotic as it can be too easy to work up an immunity to them. Trimethoprim is one that I don't handle very well though.

It helps with the symptoms of the urinary tract infection, but I end up feeling light headed and my stomach hurts after a couple days.

With so many other choices of antibiotics out there to choose from that don't give me these side effects, I will use something other than Trimethoprim.

For me, I have found that Bactrim works as well as anything, and doesn't have the bad side effects.


@ddljohn-- I know how you feel, I had the same side effects when I was taking trimethoprim for a bad case of flu. I didn't feel as sick later in the day, but would feel really sick in the mornings.

I don't know about the flu symptoms since I actually had the flu but I know that the nausea was caused by trimethoprim. Taking it with food definitely helps. It also helps to break up the tablets and take it in small amounts throughout the day. It's less harsh on the stomach that way.

I was taking two tablets a day, and I would break each into half and take each piece with a small meal throughout the day. That helped with the nausea. I hope you feel better soon!


I've been taking trimethoprim for a urinary tract infection and it makes me feel really nauseous, especially in the mornings and in the evening. I always have it right after food, but it still makes me sick like I'm about to throw up and have a fever but I actually don't. I also feel very tired and feel like resting all day long.

I'm guessing all of these are caused by the trimethoprim because I didn't have these side effects before I was prescribed them.

Does anyone else feel this way with trimethoprim? Does anything help or make you feel better?

I have to keep taking it for another five days and I'm not looking forward to it.


I was prescribed trimethoprim twice. The first time, I don't remember any side effects but I didn't use it for long. My infection didn't improve at all with it so my doctor switched me to another antibiotic.

Several years later, I got a bad gastrointestinal infection while traveling and was prescribed trimethoprim again. I used it for longer this time and it did treat my infection but I got a horrible rash all over my body. It was really frustrating and painful.

I'm honestly not sure why this antibiotic is used. It does work well in some cases, but there are so many possible side-effects involved. Aren't there enough of other alternative antibiotics that can be used instead of trimethoprim?

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