Trimethoprim: What it treats and how to use it

Trimethoprim is an antibiotic you’ll commonly hear about for urinary tract infections (UTIs) and some other bacterial problems. It works by blocking a bacterial enzyme called dihydrofolate reductase, which slows bacteria growth so your immune system can handle the rest. You’ll often see it used alone or combined with sulfamethoxazole (that combo is known as co-trimoxazole or TMP-SMX).

Compared with broad wording, here’s the practical bit: for uncomplicated UTIs in adults a common regimen is 200 mg twice daily for three days, but some infections need different doses or a longer course. Never change dose or stop early without talking to your prescriber—finishing the prescribed course helps avoid resistance.

What to expect and common side effects

Most people tolerate trimethoprim fine. The typical side effects are nausea, headache, and a mild rash. Some people may notice changes in taste or mild dizziness. Less common but more serious issues include allergic reactions, severe skin rashes, and blood changes like low white cells or anemia. Because it affects folate pathways, it can cause or worsen low blood counts in people with folate deficiency.

Trimethoprim can raise potassium levels in some people. If you’re on medications that also raise potassium (like spironolactone, ACE inhibitors, or certain blood pressure drugs), or if you have kidney disease, your doctor may check your blood tests while you’re on it.

When to avoid it and drug interactions

Avoid trimethoprim in pregnancy unless your doctor says it’s absolutely necessary—it can interfere with folate and is usually not a first choice in early pregnancy. If you’re breastfeeding, discuss risks and benefits with your provider. Also tell your prescriber if you take blood thinners (warfarin), methotrexate, or some diabetes medicines—trimethoprim can change how these drugs act or how your blood tests read.

If you get a rash, high fever, persistent sore throat, yellowing of the skin, dark urine, or flu-like symptoms after starting the drug, stop use and contact your doctor right away. Those can be signs of rare but serious reactions.

Antibiotic resistance is real. If symptoms don’t get better within 48–72 hours, go back to your clinician. They may need to test the urine, switch antibiotics, or check for another cause. For recurrent UTIs, your provider may suggest different testing or prevention steps.

Simple tips: take doses evenly spaced, swallow tablets with water, and store them at room temperature away from moisture. If you miss a dose, take it as soon as you remember unless it’s almost time for the next one—don’t double up.

Have questions about alternatives, combinations, or interactions? Talk with your pharmacist or doctor—especially if you’re pregnant, have kidney problems, or take other medicines. Trimethoprim can be very effective when used correctly, but safe use means good communication with your healthcare team.