Once a licensed physician determines you have acute bacterial sinusitis, the question becomes which antibiotic to use. This is not a one-size-fits-all decision — your allergy history, recent antibiotic exposure, and local resistance patterns all factor in. Below is what the IDSA acute bacterial rhinosinusitis guidelines and the AAO-HNS adult sinusitis guidelines recommend, and what CodyMD physicians prescribe in practice.

First-line: amoxicillin-clavulanate (Augmentin)

The IDSA's first-line recommendation for adult acute bacterial sinusitis is amoxicillin-clavulanate (Augmentin) 875 mg / 125 mg twice daily for 5–7 days. Amoxicillin inhibits bacterial cell wall synthesis by blocking the cross-linking of peptidoglycans — the same mechanism as penicillin, the antibiotic class it belongs to. Clavulanate is added because many strains of H. influenzae and Moraxella catarrhalis produce beta-lactamase enzymes that would otherwise inactivate amoxicillin. The combination covers the three main bacterial sinusitis pathogens — S. pneumoniae, H. influenzae, and M. catarrhalis.

Common side effects: diarrhea (most common, 15–25%), nausea, vomiting, rash, and vaginal yeast infection. Rare but serious adverse effects include Clostridioides difficile colitis and hepatotoxicity. Take with food to reduce GI upset.

Penicillin-allergic patients: doxycycline

For patients with a true penicillin allergy, the IDSA recommends doxycycline 100 mg twice daily (or 200 mg once daily) for 5–7 days. Doxycycline is a tetracycline that inhibits bacterial protein synthesis by binding the 30S ribosomal subunit. It has good coverage of the major sinus pathogens and reasonable tissue penetration.

Common side effects: photosensitivity (use sunscreen), nausea, esophageal irritation (take with full glass of water, sit upright 30 minutes after), and tooth discoloration in children under 8 (so it's avoided in young pediatrics). Doxycycline is also avoided in pregnancy.

Treatment failure or risk for resistance: levofloxacin

When first-line therapy fails or the patient has risk factors for resistant pathogens, a respiratory fluoroquinolone such as levofloxacin 500 mg daily for 5–7 days is the IDSA's recommended second-line option. Levofloxacin inhibits bacterial DNA gyrase and topoisomerase IV. It's effective, but the FDA has issued black-box warnings for tendon rupture, peripheral neuropathy, CNS effects, and aortic aneurysm, so it's reserved for cases where the benefit outweighs these risks.

What licensed doctors don't typically prescribe for sinus infection

Macrolides (azithromycin) are no longer recommended as first-line for acute bacterial sinusitis. Resistance rates of S. pneumoniae to macrolides exceed 30% in many US communities, making clinical failure too likely. Trimethoprim-sulfamethoxazole has similarly fallen out of favor for the same reason.

There are also no over-the-counter antibiotics in the United States — any antibiotic for sinus infection requires a prescription. Anyone offering OTC "antibiotics" is selling something else (often herbal preparations with no clinical evidence of antibacterial efficacy).

How CodyMD doctors choose the right antibiotic

When you text Cody about sinus symptoms, the licensed physician evaluating your case considers your allergies (especially penicillin and sulfa), your recent antibiotic use (which may signal resistance), pregnancy status, age, and any contraindications (kidney function, drug interactions, tendon issues). They prescribe the antibiotic with the best safety/efficacy match for your specific situation. For details on the visit, see how CodyMD treats sinus infections online.

Critically, your physician will only prescribe if your case meets the IDSA criteria for bacterial sinusitis. If you have viral sinusitis, the right answer is supportive care — see our guide to bacterial vs. viral sinusitis for the diagnostic logic.

How long until you feel better

Most patients on appropriate antibiotic therapy notice improvement within 48–72 hours. If you're not improving by day 3 of antibiotics, the diagnosis may be wrong (viral after all), the antibiotic may not be covering the right pathogen, or a complication may be developing. CodyMD's licensed doctors include a follow-up window at no additional cost — text back if you're not getting better. For symptom recognition timelines, see sinus infection symptoms.

Bottom line on sinus infection antibiotics

First-line is amoxicillin-clavulanate (Augmentin) for 5–7 days. Doxycycline for penicillin-allergic patients. Levofloxacin for treatment failure. The right antibiotic depends on you, not just the diagnosis — which is why a licensed physician makes the call. Text CodyMD; if it's bacterial, your prescription is at your pharmacy in 1 hour.