The three first-line antibiotics for a straightforward urinary tract infection are nitrofurantoin, trimethoprim-sulfamethoxazole (TMP-SMX), and fosfomycin. Which one your provider picks depends on local resistance patterns, your health history, and whether you’re pregnant. Most uncomplicated UTIs clear within three to five days of starting treatment.
First-Line Antibiotics for Simple UTIs
A simple (uncomplicated) UTI means a bladder infection in an otherwise healthy person with no structural abnormalities in the urinary tract. The 2025 guidelines from the American Urological Association recommend three options as first choices:
- Nitrofurantoin: 100 mg twice daily for 5 days
- TMP-SMX: one double-strength tablet twice daily for 3 days
- Fosfomycin: a single 3-gram dose, taken once
TMP-SMX has the shortest course at just three days, while nitrofurantoin requires five. Fosfomycin is the simplest option: you dissolve a packet of granules in a few ounces of cold water and drink it once. That single dose is the entire treatment. Clinical trials have found that five days of nitrofurantoin produces results equivalent to three days of TMP-SMX for bladder infections.
Your provider won’t just pick one at random. The choice hinges on which bacteria are most common in your area and how resistant they are to each drug. In U.S. women, roughly 19% of the E. coli strains that cause UTIs are resistant to TMP-SMX. Nitrofurantoin has lower resistance rates, around 7% in women without a history of repeat infections, which is one reason many clinicians reach for it first.
Why Fluoroquinolones Are No Longer First Choice
Ciprofloxacin and levofloxacin used to be commonly prescribed for UTIs. The FDA now advises against using them for uncomplicated infections when other options exist. These drugs carry a risk of serious side effects involving the tendons, muscles, joints, nerves, and central nervous system. Some of these effects can be disabling and permanent. For a bladder infection that nitrofurantoin or TMP-SMX can handle, the risks of fluoroquinolones simply aren’t justified.
Fluoroquinolones still have a role in more serious infections, like kidney infections or complicated UTIs, particularly when other antibiotics won’t work against the specific bacteria involved.
Second-Line Options
If you’re allergic to first-line drugs or if the bacteria are resistant to them, providers turn to beta-lactam antibiotics. These include cephalexin and amoxicillin-clavulanate. They’re generally considered slightly less effective for UTIs than the first-line options, which is why they’re held in reserve. In studies comparing amoxicillin-clavulanate to cephalexin, cure rates at two weeks were 77% and 74% respectively, dropping to 76% and 60% at six weeks.
Antibiotics for Complicated or Severe UTIs
A “complicated” UTI means the infection involves the kidneys (pyelonephritis), occurs alongside a structural problem like kidney stones, or develops in someone with a catheter or weakened immune system. These infections need stronger antibiotics and often a longer course.
For complicated UTIs without sepsis, guidelines recommend starting with a third- or fourth-generation cephalosporin, piperacillin-tazobactam, or a fluoroquinolone. When sepsis is present, carbapenems may also be used from the start. Treatment typically begins with IV antibiotics in a hospital or urgent care setting. Once you’re improving, no longer feverish, and able to eat and drink normally, your care team will switch you to an oral antibiotic for the remainder of the course. Research supports this transition even in patients with bacteria in the bloodstream, as long as improvement is clear and an effective oral option exists.
UTI Treatment During Pregnancy
UTIs in pregnancy are treated more aggressively because untreated infections can lead to kidney infections and complications for the pregnancy. The American College of Obstetricians and Gynecologists recommends a 5 to 7 day course, longer than the standard treatment for non-pregnant patients.
Safe options during pregnancy include nitrofurantoin, beta-lactam antibiotics (like amoxicillin-clavulanate and cephalexin), and fosfomycin. TMP-SMX is generally avoided, particularly in the first trimester, due to concerns about fetal development. Your provider will choose based on urine culture results and the drug’s safety profile for your stage of pregnancy.
Preventing Recurrent Infections
If you get three or more UTIs in a year, or two within six months, you may be a candidate for preventive antibiotic therapy. There are two main approaches.
Continuous daily prophylaxis uses a low dose of an antibiotic taken every day. Options include nitrofurantoin at 50 to 100 mg daily, TMP-SMX at a single-strength dose daily or three times a week, cephalexin at 125 to 250 mg daily, or fosfomycin as a single packet every 10 days. These doses are much lower than what you’d take to treat an active infection.
If your UTIs are linked to sexual activity, a different strategy works: taking a single low dose of an antibiotic right before or after intercourse. Options for this approach include TMP-SMX, nitrofurantoin (50 to 100 mg), or cephalexin (250 mg). This approach uses less total medication than daily prophylaxis while still reducing infection rates significantly.
Who Should Avoid Certain Antibiotics
Nitrofurantoin doesn’t work well when kidney function is reduced because the drug needs to concentrate in the urine to kill bacteria. It’s contraindicated when your kidney filtration rate (eGFR) falls below 45. A short course may still be used cautiously with an eGFR between 30 and 44, but only when the bacteria are resistant to other options and the benefits clearly outweigh the risks.
TMP-SMX can interact with several common medications and isn’t appropriate during early pregnancy. Fosfomycin is only approved for bladder infections, not kidney infections, because it doesn’t reach high enough concentrations in kidney tissue. Each first-line antibiotic has its own set of limitations, which is why providers need to match the drug not just to the bacteria but to you specifically.
Why Resistance Patterns Matter
The bacteria causing your UTI may already be resistant to one or more antibiotics, especially if you’ve had recent infections or recent antibiotic use. Among women with recurrent UTIs, resistance to TMP-SMX climbs to about 22%, and resistance to nitrofurantoin rises to 12%. Fluoroquinolone resistance in the recurrent group reaches nearly 16%.
This is why urine cultures matter. A culture identifies exactly which bacterium is causing the infection and which drugs can kill it. For a first, uncomplicated UTI, providers often start treatment based on the most likely cause (E. coli accounts for the vast majority) and switch if the culture results suggest a better option. For recurrent or complicated infections, waiting for culture results before choosing an antibiotic leads to more reliable outcomes.