Trimethoprim Dosage: What You Need to Know About Dosing, Uses, and Risks

When treating a trimethoprim, a synthetic antibiotic used to fight bacterial infections, especially urinary tract infections. Also known as TMP, it works by stopping bacteria from making folic acid, which they need to grow. It’s often paired with sulfamethoxazole as co-trimoxazole, but standalone trimethoprim is still widely used for simple UTIs and some skin infections. Getting the trimethoprim dosage right matters—too little won’t clear the infection, too much can cause side effects like nausea, rash, or even low blood cell counts.

For most adults with a simple urinary tract infection, the standard dose is 100 mg twice a day for three days. If it’s a more serious or recurring infection, your doctor might extend that to seven days or bump the dose to 200 mg twice daily. Kids get doses based on weight—usually 3 to 6 mg per kilogram per day, split into two doses. Elderly patients or those with kidney problems often need lower doses because trimethoprim leaves the body through the kidneys. If you’re on dialysis or have chronic kidney disease, your doctor will adjust the dosage carefully. It’s not for everyone—people with folate deficiency, severe liver disease, or a history of blood disorders should avoid it unless closely monitored.

Trimethoprim is often confused with other antibiotics like cefadroxil, a first-generation cephalosporin used for skin and urinary infections or minocycline, a tetracycline antibiotic better suited for acne and respiratory infections. But trimethoprim has a narrower range—it’s mostly for gram-negative bacteria like E. coli in the bladder. It’s not a go-to for strep throat or sinus infections. You’ll find it in many of the posts here because it’s common, affordable, and frequently prescribed. But misuse is a real problem. Taking it without a confirmed bacterial infection, skipping doses, or stopping early can lead to resistance. That’s why doctors are more cautious now than they were 10 years ago.

Some people take trimethoprim long-term to prevent recurrent UTIs—usually 50 to 100 mg once daily at bedtime. This isn’t something you start on your own. It requires regular blood tests to check for low white blood cells or anemia. Pregnant women can use it in the second and third trimesters, but it’s avoided in early pregnancy because it can interfere with folic acid, which is critical for fetal development. If you’re on birth control, trimethoprim doesn’t affect it, but it can make your skin more sensitive to sunlight. Wear sunscreen.

What you’ll find below are real, practical comparisons and guides from people who’ve used trimethoprim—or had to choose between it and other options. Some posts dig into how it stacks up against other antibiotics for UTIs. Others explain how to spot signs it’s not working, what to do if you miss a dose, or how to reduce side effects. You’ll also see how it fits into broader antibiotic use patterns, what alternatives exist when it fails, and why some clinics now avoid it entirely due to rising resistance. This isn’t just a dosage chart. It’s a real-world look at how this drug works, who it helps, and when it’s better to pick something else.