Amoxicillin for Sinus Infection: When It's Prescribed, How Fast It Works and Side Effects

Written by Cherlie Joseph, MSN, FNP-BC · Medically reviewed by Naurah Gaspard, DNP, FNP-C, FNP-BC · Updated November 2026

Amoxicillin, alone or combined with clavulanate (Augmentin), is the first-choice antibiotic for adults with a bacterial sinus infection, usually for 5 to 7 days. Most people start feeling better within 2 to 3 days. But most sinus infections are viral, so amoxicillin only helps when symptoms point to a bacterial infection.

When amoxicillin is (and isn't) the right call

Antibiotics kill bacteria. They do nothing against the viruses that cause colds and most sinus infections. The CDC notes that most sinus infections get better on their own without antibiotics.

The 2025 adult sinusitis guideline from the American Academy of Otolaryngology–Head and Neck Surgery (AAO-HNS) suggests a bacterial infection when:

• Symptoms such as congestion, thick discolored drainage and facial pain or pressure last at least 10 days without improving, or
• Symptoms get worse again within 10 days after you had started to feel better.

Even then, the guideline says many healthy adults with uncomplicated symptoms can choose watchful waiting with a plan to follow up, since many bacterial sinus infections clear without treatment. If you and your provider decide an antibiotic is worthwhile, amoxicillin with or without clavulanate for 5 to 7 days is the recommended starting point for most adults.

Amoxicillin vs Augmentin

	Amoxicillin	Amoxicillin-clavulanate (Augmentin)
What it is	A penicillin-type antibiotic	Amoxicillin plus clavulanate, which blocks an enzyme some bacteria use to resist amoxicillin
Often chosen when	Uncomplicated infection, no recent antibiotics	Recent antibiotic use, more severe symptoms, or higher risk of resistant bacteria
Stomach side effects	Possible	Diarrhea is more common; taking it with a meal can help
Cost	Low-cost generic	Generic available; usually costs somewhat more

Your provider picks between them, and the dose, based on your symptoms, recent antibiotic use, kidney function and other health factors. Always take the dose on your prescription label.

How fast it works and what if it doesn't

Many people notice less pressure and thinner drainage within 2 to 3 days. Congestion and a mild cough may take longer to fully clear. The AAO-HNS guideline advises checking back with your provider if you are not improving, or are getting worse, after 3 to 5 days on an antibiotic. That may mean the infection is viral after all, something else is going on, or a different antibiotic is needed.

Keep taking the medicine for the full course you were prescribed, even after you feel better, unless your provider tells you to stop. Don't save leftover pills or share them; a future illness may need a different treatment.

Side effects and interactions

According to MedlinePlus, common side effects include nausea, vomiting and diarrhea. Amoxicillin can be taken with food to reduce stomach upset. Antibiotics can also trigger a vaginal yeast infection in some women.

Stop the medicine and get medical help right away for:

• Hives, itching, a spreading rash, or blistering or peeling skin
• Wheezing, trouble breathing or swallowing, or swelling of the face, lips, tongue or throat
• Severe watery or bloody diarrhea with fever or cramps, which can start during treatment or even weeks after (a sign of C. difficile infection)

MedlinePlus advises that amoxicillin may make hormonal birth control less effective and suggests using a backup method while taking it; ask your provider what's right for you. Tell your provider about every medicine and supplement you take, including blood thinners.

If you're allergic to penicillin

Amoxicillin is a penicillin, so it isn't used if you have a true penicillin allergy. Guidelines list doxycycline as a common alternative for adults, and other options exist for specific situations. Older choices such as azithromycin and sulfamethoxazole-trimethoprim are generally not recommended for sinus infections because many sinus bacteria are now resistant to them.

Many people labeled "penicillin allergic" turn out not to be after testing. If your reaction was years ago or mild, such as an upset stomach, mention the details; it may open up safer, more effective options.

Frequently asked questions

How many days of amoxicillin for a sinus infection?

For most adults, the AAO-HNS guideline recommends 5 to 7 days. Your provider may choose a different length based on your situation; follow your prescription.

Can I get amoxicillin for a sinus infection online?

In Florida, a licensed nurse practitioner can evaluate sinus symptoms through a telehealth visit and prescribe an antibiotic when it is medically appropriate. A prescription is never guaranteed; if your symptoms look viral, you'll get a care plan for relief instead.

Is amoxicillin or Augmentin better for a sinus infection?

Neither is better for everyone. Augmentin covers more resistant bacteria but causes more diarrhea. Providers often choose it after recent antibiotic use or for more severe symptoms.

Sources

• AAO-HNS: Clinical Practice Guideline: Adult Sinusitis Update (2025) summary
• MedlinePlus: Amoxicillin
• American Family Physician: IDSA Guidelines for Acute Bacterial Rhinosinusitis

Keep reading

• Do you need antibiotics for a sinus infection?
• How long does a sinus infection last?
• Sinus pressure relief: 9 ways to feel better
• Browse all conditions and treatments A to Z

This article is for general education and is not a substitute for medical advice. Treatment, including any prescription, is decided by a licensed provider after reviewing your health history. OurTopClinic is not for emergencies; if you have a medical emergency, call 911. OurTopClinic serves adults located in Florida, New Mexico, Oregon and Washington. Learn more at ourtopclinic.com.