The first-line antibiotic for a bacterial sinus infection is amoxicillin-clavulanate, a combination of amoxicillin plus an ingredient that helps it work against resistant bacteria. A standard course lasts 5 to 7 days for most adults. But most sinus infections are viral, not bacterial, which means antibiotics won’t help unless specific symptoms point to a bacterial cause.
Most Sinus Infections Don’t Need Antibiotics
Only about 0.5% to 2% of sinus infections turn bacterial. The rest are caused by viruses, and no antibiotic will shorten a viral infection. This is why doctors don’t prescribe antibiotics right away, even when your symptoms feel miserable.
Antibiotics become appropriate when symptoms fit one of three patterns: your congestion, facial pain, or nasal discharge persist for 10 days or more without any improvement; you develop a high fever (102°F or above) along with purulent nasal discharge and facial pain lasting at least 3 to 4 consecutive days; or your symptoms start to get better after a week, then suddenly worsen again. That last pattern, sometimes called “double sickening,” is a strong signal that a bacterial infection has set in on top of the original viral one.
Amoxicillin-Clavulanate: The Standard Choice
Guidelines from the Infectious Diseases Society of America recommend amoxicillin-clavulanate over plain amoxicillin for adults with bacterial sinusitis. The clavulanate component blocks an enzyme that certain bacteria use to resist amoxicillin, making the combination effective against a broader range of organisms.
For uncomplicated cases, a 5 to 7 day course is the standard recommendation. If you’re at higher risk for antibiotic resistance (you’ve taken antibiotics in the past month, you’re over 65, you’re immunocompromised, or you live in an area with high rates of resistant bacteria), your doctor may prescribe a higher dose version and extend treatment to 7 to 10 days.
The most common side effect is digestive upset, particularly diarrhea. Taking it at the start of a meal or snack helps reduce stomach problems. If you develop watery or bloody diarrhea, even weeks after finishing the course, that warrants a call to your doctor since it can signal a more serious gut reaction.
Options if You’re Allergic to Penicillin
Amoxicillin-clavulanate is a penicillin-based drug, so it’s off the table if you have a penicillin allergy. The go-to alternative is doxycycline, typically taken twice daily. It covers the main bacteria responsible for sinus infections and works well for most adults.
If you can tolerate cephalosporins (a related but distinct class from penicillin, which many penicillin-allergic patients can safely take), a third-generation cephalosporin like cefpodoxime or cefixime is another option. These are sometimes combined with clindamycin for broader coverage. For more severe allergies or treatment failures, respiratory fluoroquinolones like levofloxacin or moxifloxacin are reserved as backup options due to their stronger side effect profile.
Why Z-Paks Are No Longer Recommended
Azithromycin, the antibiotic in a Z-Pak, was once a popular choice for sinus infections. It’s convenient (a short course, once-daily dosing) and well tolerated. The problem is that the bacteria most commonly behind sinus infections have developed widespread resistance to it.
A large U.S. study across 329 hospitals found that roughly 40% of Streptococcus pneumoniae isolates, one of the primary bacteria in sinusitis, were resistant to macrolides like azithromycin. Among respiratory samples specifically, resistance climbed to 47%. Guidelines now state that macrolide monotherapy should only be used when local resistance rates are below 25%, a threshold most U.S. regions exceed. In practical terms, prescribing a Z-Pak for a bacterial sinus infection means close to a coin flip on whether it will actually work.
Antibiotics for Children
Children follow a similar approach, with amoxicillin-clavulanate as the first choice. The standard dose is based on body weight: 45 mg per kilogram per day of the amoxicillin component, split into two doses. When antibiotic resistance is a concern or the child has severe symptoms, that dose doubles to 90 mg per kilogram per day.
For children with penicillin allergies, third-generation cephalosporins or clindamycin are the typical alternatives. Doxycycline is generally avoided in younger children due to effects on developing teeth, though it’s considered safe for kids over eight.
Chronic Sinusitis Is Treated Differently
Chronic sinusitis, where symptoms persist for 12 weeks or longer, follows a different playbook than an acute infection. Antibiotics may help during acute flare-ups of chronic sinusitis, but there’s no strong consensus on how long to prescribe them. Most doctors use courses ranging from 7 days up to 4 weeks depending on the severity and response.
The antibiotic choices are similar: a penicillin-based drug as first line, with doxycycline or fluoroquinolones for those who can’t take penicillin. One notable difference is that low-dose, long-term macrolide or doxycycline therapy (up to 12 weeks) has shown some benefit in chronic sinusitis with nasal polyps. A 20-day course of doxycycline, for example, can reduce polyp size and lower inflammation markers in nasal secretions. This longer-term use is specific to chronic sinusitis and isn’t relevant for a standard acute infection.
What to Expect During Treatment
Most people notice improvement within 3 to 5 days of starting antibiotics. If your symptoms haven’t improved at all after 3 to 5 days, contact your doctor, as you may need a different antibiotic or a reassessment of whether the infection is truly bacterial.
Finish your entire prescribed course even if you start feeling better early. Stopping short increases the chance of relapse and contributes to antibiotic resistance. For uncomplicated acute sinusitis, that full course is usually just 5 to 7 days, shorter than many people expect. In fact, research has found that many prescriptions in the U.S. still default to 10-day courses despite guidelines recommending shorter treatment for most patients. A shorter course works just as well for low-risk cases and reduces your exposure to side effects.