Amoxicillin for Sinus Infection, Ear Infection, and Strep Throat: When It Works and When It Doesn't

Amoxicillin for sinus infection is not the guideline first choice. IDSA recommends amoxicillin-clavulanate (Augmentin) instead, because a large share of Haemophilus influenzae strains resist plain amoxicillin. Plain amoxicillin does stay first-line for ear infections, at 80 to 90 mg/kg/day, and for strep throat confirmed by a test.

TL;DR: Key takeaways

• Most sinus and ear infections are viral and clear without any antibiotic.
• For bacterial sinusitis, guidelines favor amoxicillin-clavulanate over plain amoxicillin.
• Ear infections are the exception: high-dose plain amoxicillin, 80 to 90 mg/kg/day, is still first line.
• Strep needs a positive test first, then a full 10 days of treatment.
• Expect improvement in 24 to 48 hours for strep, and 2 to 4 days for sinus and ear infections.

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Plain amoxicillin for sinus infection usually isn't the right first choice

Amoxicillin for sinus infection is prescribed constantly, but it is not what the guidelines actually put first. The 2012 IDSA guideline for acute bacterial rhinosinusitis names amoxicillin-clavulanate, not amoxicillin alone, as first-line empiric therapy in both children and adults. The reason is narrow and specific: 27% to 43% of H. influenzae isolates are amoxicillin-resistant but still susceptible when clavulanate is added.

Clavulanate is not a stronger antibiotic. It is a shield. It blocks the enzyme those bacteria use to dismantle amoxicillin before the drug can do anything useful.

Here is what most pages will not tell you: plenty of experienced clinicians still write plain amoxicillin for sinusitis, and for an uncomplicated case in an otherwise healthy adult who has not taken antibiotics recently, it frequently works. The prescriber is weighing local resistance patterns, your allergy history, and what you have already been treated with this year. A prescription for plain amoxicillin is not a mistake. It is a judgment call you are entitled to ask about.

Most sinus and ear infections never needed an antibiotic in the first place

The CDC is blunt about this: most sinus infections are caused by viruses and improve on their own. Clinicians may use watchful waiting, or write a delayed prescription that you fill only if things have not improved in two or three days. Rest, fluids, and saline rinses are doing the work in the meantime.

IDSA describes three patterns that suggest bacteria rather than a virus:

• Symptoms lasting 10 days or longer with no improvement.
• Severe onset: fever of 39°C (102°F) or higher plus purulent discharge or facial pain, for 3 to 4 consecutive days.
• "Double-sickening": you started getting better, then clearly got worse.

Sore throat works differently but lands in the same place. The CDC states that clinical exam alone cannot distinguish viral pharyngitis from group A strep. A positive rapid antigen test or throat culture comes first, and a negative rapid test in a child should be confirmed with a culture. Do not expect or request amoxicillin on symptoms alone. Pressure at the front desk is how a viral illness ends up with a side of diarrhea and no benefit. If your symptoms are unusual, such as a sinus infection without congestion, that ambiguity matters even more.

Sinus dosing splits by age: a shorter course for adults, a longer one for kids

The amoxicillin dosage for sinus infection is not one fixed number, and course length tracks age more than severity. IDSA sets 5 to 7 days for adults with uncomplicated acute bacterial rhinosinusitis and 10 to 14 days for children. High-dose regimens (2 g twice daily in adults, 90 mg/kg/day in children) are reserved for risk factors: recent antibiotic use, day-care attendance, or a region with known resistance concerns.

Standard adult bands seen in practice are 500 mg every 8 hours or 875 mg every 12 hours. Our fuller breakdown of the amoxicillin dose for adults covers how those bands shift by indication.

For children, the amoxicillin dosage for sinus infection is weight-based, which has a practical consequence parents underestimate: the same child's correct dose changes between illnesses as they grow. Last winter's leftover bottle is last winter's dose.

Ear infections are the one place high-dose amoxicillin is still first-line

Amoxicillin for ear infection is the mirror image of the sinus story, because here plain amoxicillin is still the guideline answer. The 2013 AAFP rapid evidence review of acute otitis media puts high-dose amoxicillin, 80 to 90 mg/kg/day in two divided doses, first for children.

Duration splits by age and severity: 10 days for children under 2 or with severe symptoms, and 5 to 7 days for mild cases in children 2 and older. Observation without antibiotics is a legitimate option in selected non-severe cases when follow-up is assured, which is worth raising if your child has had repeated courses.

Adults get ear infections too. They are treated with standard adult amoxicillin dosing rather than a weight-based calculation. If infections keep returning, read up on ear infections and hearing problems before the next round.

A strep prescription is about preventing rheumatic fever, not soothing your throat

Amoxicillin for strep throat is prescribed for a reason most people never hear stated out loud. The CDC is explicit that the point of treating is to lower the risk of acute rheumatic fever and suppurative complications such as peritonsillar abscess and mastoiditis, not simply to shorten a sore throat by a day.

The CDC first-line regimen is amoxicillin 50 mg/kg once daily, to a maximum of 1,000 mg, for 10 days. Penicillin V is equally effective and is preferred in some stewardship protocols because it is narrower-spectrum, which is why guidance occasionally reads as though amoxicillin is the runner-up. It is not. It is the more palatable option, especially for children.

Finish all 10 days even after the throat feels normal on day three. Once fever-free, return to school, work, or daycare 24 hours after the first dose.

Feeling better in two days doesn't mean you can stop the course

How long does amoxicillin take to work depends on which infection is being treated. Strep symptoms often ease within 24 to 48 hours. Sinus and ear infections usually take 2 to 4 days to turn the corner, because inflamed, blocked spaces drain slowly even after the bacteria are losing.

No meaningful improvement by 48 to 72 hours is a phone call, not a wait-and-see. Three explanations are on the table: a resistant organism, the wrong diagnosis, or a viral illness that was never going to respond.

The catch: do not judge success by whether your head feels clear. Fluid behind the eardrum and post-infection congestion can persist for weeks after the infection itself is treated. That lingering fullness is drainage, not failure, and it is not a reason for a second antibiotic.

Some amoxicillin reactions mean call the office; others mean call 911

The common amoxicillin side effects are unglamorous and usually harmless: diarrhea, nausea, a mild rash, and yeast infections. They are annoying, not dangerous, and most do not require stopping. A short list of reactions is genuinely different.

Call 911 or your local emergency line or go to an emergency room now if:

• You have wheezing, difficulty breathing or swallowing, or swelling of the face, throat, tongue, lips, or eyes, or severe skin blistering or peeling.
• You develop severe watery or bloody diarrhea with fever and cramps, which can signal C. difficile colitis.
• A sinus infection comes with severe or worsening headache, facial or eye swelling, vision change, or confusion.
• A sore throat comes with drooling, a muffled "hot potato" voice, or inability to open the mouth.
• A child has swelling, redness, or pain behind the ear along with fever.

Book a visit within the next week if:

• Sinus symptoms have run 10 days or more without improvement.
• You are no better after 3 days on the antibiotic.
• You get four or more sinus episodes a year.
• Ear infections keep repeating, or hearing seems off after treatment.

Talk to a clinician before you start if:

• You have a penicillin or cephalosporin allergy, kidney disease, or a prior C. difficile infection.
• Mononucleosis is suspected.
• You are pregnant or breastfeeding.
• You take methotrexate or allopurinol.

If you are unsure which tier you are in, the free August AI Symptom Checker will help you sort urgent from routine before you decide where to go.

The bottom line on amoxicillin for sinus, ear, and strep infections

The diagnosis threshold comes before the prescription, every time. For sinus infections, amoxicillin-clavulanate is favored over plain amoxicillin, for 5 to 7 days in adults and 10 to 14 days in children. For ear infections, high-dose plain amoxicillin at 80 to 90 mg/kg/day remains first-line. For confirmed strep, it is 50 mg/kg once daily, up to 1,000 mg, for a full 10 days.

Finish the course even after you feel well, and call back if there is no improvement by 72 hours.