The first-line antibiotic for a bacterial sinus infection in adults is amoxicillin-clavulanate, a combination of amoxicillin plus an ingredient that helps it work against resistant bacteria. But here’s the important caveat: most sinus infections are viral, not bacterial, and antibiotics won’t help a viral infection at all. The antibiotic your doctor prescribes (or whether they prescribe one) depends on how long you’ve been sick, how your symptoms have progressed, and whether you have drug allergies.
Most Sinus Infections Don’t Need Antibiotics
The vast majority of sinus infections start as viral illnesses, essentially a cold that settles into your sinuses. These clear up 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 is diagnosed based on a specific pattern of symptoms. Clinicians look for purulent (thick, discolored) nasal drainage along with nasal obstruction or facial pain and pressure. The key factor is timing: symptoms must persist without any improvement for at least 10 days, or they must worsen after an initial period of getting better. That second pattern, sometimes called “double sickening,” is one of the strongest signals that bacteria have moved in. You start to feel better, then suddenly get worse again with worsening congestion, new fever, or intensifying facial pain.
If your symptoms haven’t hit either of those thresholds, you likely have a viral infection and won’t benefit from antibiotics.
First-Line Antibiotic for Adults
When a bacterial sinus infection is confirmed, the Infectious Diseases Society of America recommends amoxicillin-clavulanate as the go-to treatment for adults. This is preferred over plain amoxicillin because the clavulanate component overcomes a common defense mechanism that certain sinus bacteria use to resist amoxicillin alone. The two bacteria most often responsible for bacterial sinusitis are well covered by this combination.
A standard course is typically 5 to 7 days for uncomplicated cases. A CDC-backed study found that more than 20% of antibiotic prescriptions for sinus infections were unnecessarily long, so if your doctor prescribes a shorter course, that’s consistent with current evidence. Finishing the prescribed course matters, even if you start feeling better after a few days.
Why Azithromycin Is a Poor Choice
Despite being one of the most commonly prescribed antibiotics for sinus infections, azithromycin (the familiar “Z-pack”) is actually recommended against by clinical guidelines. The reason is straightforward: the bacteria that cause sinus infections have developed high rates of resistance to azithromycin and related drugs in its class. Yet studies show that over 20% of sinus infection prescriptions are still written for a 5-day course of azithromycin. If you’re offered a Z-pack for a sinus infection, it’s worth asking about amoxicillin-clavulanate instead.
Options If You’re Allergic to Penicillin
Since amoxicillin-clavulanate is a penicillin-based drug, it’s off the table if you have a penicillin allergy. The primary alternative is doxycycline, which works through a completely different mechanism and covers the same sinus bacteria effectively.
If you can tolerate cephalosporins (a class of antibiotics related to penicillin but often safe for people with mild penicillin allergies), a third-generation cephalosporin is another option, sometimes combined with a second antibiotic to broaden coverage. Your doctor can help determine whether your allergy history makes cephalosporins safe for you, since many people labeled as penicillin-allergic can actually tolerate them.
What Happens When the First Antibiotic Doesn’t Work
If you’ve been on your antibiotic for 72 hours with no improvement at all, that’s the standard window for reassessing treatment. At that point, your doctor will typically switch to a broader-spectrum option. For someone who started on amoxicillin-clavulanate, the next step may involve a fluoroquinolone antibiotic, which covers a wider range of bacteria.
However, fluoroquinolones carry an FDA boxed warning (the most serious type) specifically noting that their risks generally outweigh the benefits for acute bacterial sinusitis when other treatment options exist. These risks include potentially permanent damage to tendons, muscles, joints, and nerves. Fluoroquinolones are reserved for situations where first-line and alternative antibiotics have failed or can’t be used. They should not be a first choice for a straightforward sinus infection.
Recent antibiotic use also matters. If you’ve taken any antibiotic in the past six weeks, your doctor may start with a broader-spectrum drug from the beginning, since the bacteria in your sinuses are more likely to be resistant to first-line options.
Antibiotic Treatment for Children
For children ages 1 to 18, amoxicillin (with or without clavulanate) is also the first-line treatment, but the dosing is weight-based. For uncomplicated cases in children over age 2 who haven’t recently taken antibiotics, standard-dose amoxicillin is often sufficient. Children under 2, those in daycare, those with moderate to severe symptoms, or those who’ve recently been on antibiotics typically receive high-dose amoxicillin-clavulanate, which delivers more of the active drug to overcome resistant bacteria.
The same diagnostic criteria apply to children: symptoms lasting beyond 10 days without improvement, or a clear worsening after initial improvement. Kids get frequent colds, and most of those colds are not bacterial sinus infections, so the same caution about avoiding unnecessary antibiotics applies.
What Else Helps While You’re on Antibiotics
Antibiotics target the bacteria, but they don’t directly relieve the congestion and pressure that make sinus infections miserable. Saline nasal irrigation (using a neti pot or squeeze bottle) helps flush mucus and can speed up symptom relief. Nasal corticosteroid sprays reduce the swelling inside your nasal passages and sinuses, helping them drain. Both are commonly recommended alongside antibiotics and are low-risk. Over-the-counter pain relievers handle the facial pain and headache that often accompany a sinus infection.
Decongestant nasal sprays provide fast relief but shouldn’t be used for more than 3 days, as they can cause rebound congestion that makes things worse. Oral decongestants are an option for longer use but can raise blood pressure.