The most commonly prescribed antibiotic for a straightforward UTI (bladder infection) is nitrofurantoin, taken twice daily for five days. A close second is trimethoprim-sulfamethoxazole, often called TMP-SMX or by the brand name Bactrim, taken twice daily for three days. These two are the go-to choices for uncomplicated urinary tract infections in otherwise healthy adults, and symptoms typically start improving within a few days of the first dose.
Which one you actually get depends on local resistance patterns, your allergy history, and what other medications you take. Here’s what to know about each option and why your prescription might look different from someone else’s.
The Two First-Line Antibiotics
Nitrofurantoin works by concentrating almost entirely in the urine, which makes it effective against bladder infections while causing minimal disruption to bacteria elsewhere in your body. The standard course is 100 mg twice a day for five days (the extended-release capsule form). An older formulation uses 50 to 100 mg four times daily, but the twice-daily version is more common now because it’s easier to stick with. Nitrofurantoin has stayed effective for decades because bacteria have been slow to develop resistance to it.
Trimethoprim-sulfamethoxazole is the other top choice, and its advantage is a shorter course: one double-strength tablet twice a day for just three days. The catch is that resistance rates vary by region. Guidelines recommend it only when local resistance among UTI-causing bacteria stays below 20%. Your provider may know whether resistance is a concern in your area, or they may send a urine culture to check before prescribing it.
The Single-Dose Option
Fosfomycin is the only antibiotic for bladder infections that works as a true one-and-done treatment. It comes as a single 3-gram packet of powder that you dissolve in about half a cup of cool water and drink immediately. You take it once, and that’s the entire course. It’s convenient, and bacteria rarely develop resistance to it. The tradeoff is that it doesn’t clear infections quite as reliably as a full course of nitrofurantoin or TMP-SMX, so it’s typically reserved for situations where those options aren’t suitable.
Why You Probably Won’t Get Ciprofloxacin
Ciprofloxacin and other fluoroquinolones (like levofloxacin) used to be widely prescribed for UTIs. That’s changed significantly. The FDA now carries a boxed warning on these drugs, the most serious type of safety alert, linking them to tendon rupture, nerve damage that can be permanent, and central nervous system effects including confusion, anxiety, and hallucinations. The risk of tendon problems is higher if you’re over 60 or take corticosteroids.
Current guidelines are clear: fluoroquinolones should be reserved for more serious infections, not routine bladder infections. The FDA label specifically states that ciprofloxacin should only be used for uncomplicated UTIs when no other treatment option exists. If a provider prescribes ciprofloxacin for a simple bladder infection without trying other antibiotics first, it’s reasonable to ask why.
Backup Antibiotics When First Choices Don’t Work
If you’re allergic to first-line options or your urine culture shows a resistant strain, providers turn to a group of antibiotics called beta-lactams. These include amoxicillin-clavulanate (Augmentin), cephalexin (Keflex), cefdinir, and cefpodoxime. They work, but clinical trials consistently show lower cure rates compared to nitrofurantoin or TMP-SMX. In one study comparing amoxicillin-clavulanate to cephalexin, bacteriological cure rates at six weeks were 76% and 60% respectively. These are considered second-line options for a reason, but they’re a solid fallback when needed. Courses typically run three to seven days.
Kidney Infections Need Different Treatment
Everything above applies to uncomplicated cystitis, meaning an infection limited to the bladder. If the infection has traveled to the kidneys (pyelonephritis), the approach changes. Kidney infections cause fever, back or flank pain, nausea, and generally feel much worse than a bladder infection. Nitrofurantoin doesn’t work for kidney infections because it concentrates in the bladder and doesn’t reach kidney tissue effectively.
For kidney infections, fluoroquinolones are considered appropriate for five to seven days, since the severity justifies their risks. Non-fluoroquinolone antibiotics can also be used but typically need at least seven days. Some kidney infections require IV antibiotics in a hospital initially before switching to oral medication. Updated 2025 guidelines from the Infectious Diseases Society of America now favor shorter courses of seven days over the older standard of 10 to 14 days, as long as you’re improving on treatment.
UTI Antibiotics During Pregnancy
UTIs are common during pregnancy, and leaving them untreated raises the risk of kidney infection and preterm delivery. The safe choices are penicillin-type antibiotics (like amoxicillin) and cephalosporins (like cephalexin). These have long safety records in pregnancy.
Nitrofurantoin is generally avoided in the first trimester because of a small increased risk of cleft lip, though it’s considered safe in the second and third trimesters. TMP-SMX is also typically avoided in the first trimester because it interferes with folate metabolism. Fluoroquinolones are not used during pregnancy at all. If you’re pregnant and develop UTI symptoms, your provider will likely choose from the penicillin or cephalosporin families and may treat for a full seven days rather than the shorter courses used outside of pregnancy.
What to Expect After Starting Treatment
Most people notice their symptoms easing within two to three days of starting antibiotics. The burning during urination and the constant urge to go are usually the first things to improve. Even when you feel better quickly, finishing the full prescribed course matters. Stopping early increases the chance that some bacteria survive and the infection returns, potentially with resistance to the antibiotic that almost worked.
If your symptoms haven’t improved after two to three days on antibiotics, it likely means the bacteria causing your infection are resistant to the one you were prescribed. This is why many providers send a urine culture at your initial visit. Culture results take about 48 hours to come back, and if they show resistance, your provider can switch you to an antibiotic that matches the specific bacteria involved. Recurrent UTIs, defined as two or more in six months or three or more in a year, often warrant cultures every time to guide treatment precisely.