Amoxicillin, with or without clavulanate, is the first-line antibiotic for a bacterial sinus infection. A standard course runs 5 to 10 days. But here’s the important caveat: most sinus infections are caused by viruses, not bacteria, and antibiotics won’t help a viral infection at all. Knowing the difference between the two determines whether you actually need a prescription.
Most Sinus Infections Don’t Need Antibiotics
The vast majority of sinus infections start with a virus, the same kind that causes a common cold. Viral sinusitis causes congestion, facial pressure, thick nasal discharge, and sometimes a low-grade fever. These symptoms are miserable, but they resolve on their own within 7 to 10 days. Antibiotics do nothing against viruses, and taking them unnecessarily contributes to antibiotic resistance.
A bacterial sinus infection develops when bacteria take hold in sinuses that are already inflamed and blocked, often as a complication of a viral illness. Three bacteria are most commonly responsible: Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. These are the organisms that antibiotics target.
How to Tell It’s Bacterial
Doctors use specific clinical patterns to distinguish a bacterial sinus infection from a viral one. You likely have a bacterial infection if any one of these three scenarios applies:
- Persistent symptoms: Your congestion, facial pain, or nasal discharge has lasted 10 days or more with no improvement.
- Severe onset: You developed a high fever (102°F or higher) along with thick, discolored nasal discharge or intense facial pain, and these symptoms have been present for at least three consecutive days.
- Double-sickening: You started to feel better after a cold, then got noticeably worse again around day five or six, with returning fever, worsening headache, or increased nasal discharge.
If your symptoms don’t match any of these patterns, your sinus infection is almost certainly viral. Over-the-counter decongestants, saline rinses, and pain relievers are the appropriate treatment while your body clears it.
First-Line Antibiotic: Amoxicillin
When antibiotics are warranted, amoxicillin is the go-to choice. It’s effective against the most common sinus bacteria, inexpensive, widely available, and well-tolerated. For uncomplicated bacterial sinusitis in adults, the typical course is 5 to 10 days.
In some cases, your doctor may prescribe amoxicillin-clavulanate instead of plain amoxicillin. The added clavulanate helps the antibiotic work against bacteria that have developed the ability to break down amoxicillin on their own. This combination is more likely to be chosen if you’ve used antibiotics recently, have moderate or severe symptoms, or live in an area with higher rates of resistant bacteria. Children’s doses are calculated by body weight.
The most common side effect of amoxicillin-clavulanate is digestive upset, particularly diarrhea. The clavulanate component is usually the culprit. Taking the medication with food can help reduce stomach-related side effects.
Options If You’re Allergic to Penicillin
Amoxicillin belongs to the penicillin family, so if you have a penicillin allergy, you’ll need an alternative. The two main options are trimethoprim-sulfamethoxazole (commonly known as Bactrim) and macrolide antibiotics such as azithromycin (the well-known Z-Pak) or clarithromycin. A typical course of azithromycin for sinusitis is just three days, while clarithromycin is usually taken for 14 days and Bactrim for about 10 days.
It’s worth noting that these alternatives may not cover the common sinus bacteria as reliably as amoxicillin does. Resistance to macrolides like azithromycin has increased over the years, which is one reason they’re reserved for people who genuinely can’t take penicillin-based drugs rather than used as a first choice.
What Happens If the First Antibiotic Doesn’t Work
You should notice meaningful improvement within about 72 hours of starting an antibiotic. If your symptoms haven’t budged after three days, the initial prescription likely isn’t covering the bacteria causing your infection. At that point, your doctor will typically switch you to a stronger option.
The usual step-up is high-dose amoxicillin-clavulanate (if you started on plain amoxicillin) or a respiratory fluoroquinolone. Fluoroquinolones provide broader coverage against both S. pneumoniae and H. influenzae and are generally reserved for this second-line role because of their stronger side effect profile. If you started on doxycycline, a fluoroquinolone is also the typical next step.
Treatment failure can also look like a relapse: you finish your antibiotic course, feel better for a week or two, and then the same symptoms return. This pattern within six weeks of your original infection usually calls for the same escalation to a broader-spectrum antibiotic.
Symptoms That Need Prompt Attention
A straightforward bacterial sinus infection, while uncomfortable, responds well to the right antibiotic. Rarely, infection can spread beyond the sinuses into nearby structures like the eye socket or the tissue surrounding the brain. The CDC recommends seeing a healthcare provider if you have severe headache or facial pain, symptoms that worsen after initially improving, symptoms lasting more than 10 days without getting better, or a fever that persists beyond three to four days. Vision changes, swelling around the eyes, or a stiff neck alongside a sinus infection warrant urgent evaluation, as these can signal a complication that antibiotics alone won’t resolve.
People who experience multiple sinus infections in a single year may have an underlying issue, such as nasal polyps, a deviated septum, or allergies, that keeps the sinuses chronically inflamed and prone to bacterial overgrowth. Treating that root cause often does more to prevent future infections than repeated courses of antibiotics.