Amoxicillin (Amoxil)

  • Generic name: Amoxicillin Trihydrate
  • Trade names: Amoxil, Trimox, Ospamox
  • Dosages:
    Tablets, chewable 200 mg, 400 mg;
    Tablets 500 mg, 875 mg;
    Capsules 250 mg, 500 mg;
    Powder for oral suspension 125 mg per 5 mL; 200 mg per 5 mL ; 250 mg per 5 mL; 400 mg per 5 mL
  • Pharmacologic category: Antibiotic, Aminopenicillins
  • Pregnancy risk factor: B
Medical uses

Amoxicillin is a semisynthetic antibiotic of the aminopenicillin group. It is an analog of ampicillin.

Amoxicillin is indicated for the treatment of the infections caused by susceptible strains of specific organisms:

  • Ear, nose, and throat infections - otitis media, pharyngitis, sinusitis, laryngitis and tonsillitis.
  • Genitourinary tract infections - bacterial vaginitis, cystitis, pyelonephritis.
  • Skin and skin structure infections - cellulitis (infection of the dermis and subcutaneous tissue), erysipelas (superficial form of cellulitis), folliculitis, furuncles, carbuncles.
  • Lower respiratory tract infections - bronchitis, pneumonia.
  • Helicobacter pylori infection - duodenal ulcer caused by H. pylori (in combination with clarithromycin and a PPI)

Common side effects of amoxicillin include an upset stomach, vomiting, and diarrhea. This is primarily due to the destruction of normal flora within the gastrointestinal tract. The most severe side effects include anaphylactic reaction, pseudomembranous colitis, leukopenia, and agranulocytosis.

Pharmacological characteristics
  • Absorption: Rapidly absorbed after oral administration
  • Metabolism: Partially hepatic
  • Elimination half-life: Elimination is primarily via the kidneys and is rapid, with a relatively short half-life of approximately 1 hour.
  • Excretion: Approximately 60% excreted in the urine within 6 to 8 h as unchanged drug
  • One of the safest antibiotics. Approved for use in neonates and children. Labeled as "Pregnancy category B".
  • Amoxicillin offers an advantage over other penicillins (e.g. ampicillin) in that it is better absorbed from the gastrointestinal tract and because it provides higher and more prolonged blood antimicrobial levels.
  • It has been extensively learned in many scientific studies.
  • Amoxicillin diffuses into most body tissues and fluids, with the exception of brain and spinal fluid.
  • Amoxicillin is indicated for a broad range of infections, and is commonly prescribed as a first-line therapy for otitis media (middle ear infection), pharyngitis (sore throat), and sinusitis (sinus infection).
  • Group A streptococcus, the bacteria causing strep throat, have been uniformly susceptible to amoxicillin and have not developed resistance, despite the long-term use of amoxicillin for Streptococcal Pharyngitis.
  • Inexpensive in comparison with many other antibiotics.
  • Safe for use by pregnant and lactating women who are not allergic to this type of antibiotics.
  • Not effective against beta-lactamase-producing organisms.
  • There have been rare cases of severe allergic reactions including angioneurotic oedema, anaphylaxis, serum sickness, hypersensitivity vasculitis and interstitial nephritis.
  • Frequent gastrointestinal side effects, such as nausea, vomiting, and diarrhea.
  • A recent study in the Archives of Pediatrics and Adolescent Medicine6 found the link between amoxicillin use during infancy and developmental enamel defects of permanent teeth. However, further research is needed.
  • Laboratory test interactions. High urine concentrations of amoxicillin may result in false-positive reactions when testing for the presence of glucose in urine using CLINITEST®, Benedict’s Solution, or Fehling’s Solution.
Unlabeled uses
  • Chlamydia is a common sexually transmitted disease (STD) caused by the bacterium, Chlamydia trachomatis, which can damage a woman's reproductive organs. Amoxicillin is a recommended treatment of Chlamydia during pregnancy by Centers for Disease Control and Prevention (CDC)5.
  • Prophylaxis (prevention) of inhalational anthrax. According to the CDC, amoxicillin may be used to prevent the development of anthrax in people for whom other antibiotics are contraindicated (e.g. children and pregnant women)2.
  • Endocarditis. Amoxicillin is used for prevention of bacterial endocarditis. Endocarditis is a potentially life-threatening infection of the inner surface of the heart or the heart valves caused by bacteria usually found in the mouth, intestinal tract or urinary tract. When alpha-hemolytic streptococci are likely causes of endocarditis, the recommended standard prophylactic regimen is a single dose of oral amoxicillin.
  • Lyme disease. Lyme disease is transmitted to people by ticks. Amoxicillin is the treatment of choice for most adults and children. Amoxicillin (3-4 weeks therapy) is generally effective in early disease4.
Amoxicillin for Strep Throat

Strep throat (bacterial pharyngitis) is caused by Streptococcus pyogenes, or group A beta-hemolytic streptococci (GABHS). GABHS accounts for about 15-30% of pharyngitis cases in children and 5-10% of cases in adults.

Strep throat is contagious and is spread by via the airborne route. The presence of strep bacteria can be confirmed with a throat culture.

Although strep throats are usually mild, and would often get better on their own, some untreated strep throats result in serious complications such as rheumatic fever (joint and heart disease) and post-streptococcal glomerulonephritis (inflammation of the kidneys).

Amoxicillin is very effective in the treatment of strep throat. Streptococcus pyogenes are highly susceptible to amoxicillin and have not developed resistance to it. The IDSA's newly revised guidelines7 for Group A streptococcal pharyngitis advise that when a strep infection is confirmed by testing, it should be treated with penicillin or amoxicillin.

According to the recent data1 the rate of eradication of group A beta-hemolytic streptococci from the tonsils with amoxicillin is greater than 80%. In the US prospective observational study3 the elimination of strep throat symptoms was achieved in 84% of amoxicillin-treated children.

Dosage for adults: 500 mg twice a day for 10 days.

Dosage for children: <40 kg, 375 mg twice a day for 10 days; >40 kg, 500 mg twice a day for 10 days.

If Amoxicillin is not working for Strep Throat...
In case of amoxicillin failure to cure streptococcal pharyngitis alternative treatments must be used, e.g. cefuroxime, cefixime, cefdinir, and cefpodoxime.
Patients most likely to experience amoxicillin treatment failure are those who have recently received this drug and then take the same antibiotic again.

Mode of action

Amoxicillin has a moderate-spectrum, which includes a wide range of Gram-positive and a limited range of Gram-negative organisms.

Amoxicillin is a bactericidal antibiotic (kill the bacteria). It prevents bacterial cell wall mucopeptide synthesis by acylating the enzyme transpeptidase, thus making it unable to cross-link muramic acid containing peptidoglycan strands. This inhibition of the biosynthesis of dipeptidoglycan, a substance necessary for cell wall strength and rigidity, results in a defective cell wall.

Reviews, Discussions, Forums
  • 1. Brook I, Gober AE. Rate of eradication of group A beta-hemolytic streptococci in children with pharyngo-tonsillitis by amoxicillin. Int J Pediatr Otorhinolaryngol. 2009 May;73(5):757-9 PubMed
  • 2. Guidelines for Pregnant Women Who Have Been Exposed to Anthrax But Do Not Have Symptoms. CDC
  • 3. Curtin-Wirt C, Casey JR, Murray PC, Cleary CT, Hoeger WJ, Marsocci SM, Murphy ML, Francis AB, Pichichero ME. Efficacy of amoxicillin in children with group A beta hemolytic streptococcal tonsillopharyngitis. Clin Pediatr (Phila). 2003 Apr;42(3):219-25. SagePub
  • 4. Monsel G, Canestri A, Caumes E. Antibiotherapy for early localized Lyme disease. Med Mal Infect. 2007 Jul-Aug;37(7-8):463-72
  • 5. Sexually transmitted diseases treatment guidelines 2002. CDC.
  • 6. Hong L, Levy SM, Warren JJ, Dawson DV, Bergus GR, Wefel JS. Association of amoxicillin use during early childhood with developmental tooth enamel defects. Arch Pediatr Adolesc Med. 2005 Oct;159(10):943-8.
  • 7. Shulman ST, Bisno AL, Clegg HW, Gerber MA, Kaplan EL, Lee G, Martin JM, Van Beneden C; Infectious Diseases Society of America. Clinical practice guideline for the diagnosis and management of group A streptococcal pharyngitis: 2012 update by the Infectious Diseases Society of America. Clin Infect Dis. 2012 Nov 15;55(10) Available at

Last updated: February 10, 2015

Interesting facts


  • Amoxicillin is usually very safe. The greatest risk is an allergic reaction, which can be severe.
  • Children are more likely to show allergy to amoxicillin on penicillin skin testing than adults, according to a study presented at the American College of Allergy, Asthma & Immunology 2010 Annual Scientific Meeting.
  • The narrow spectrum of activity of the penicillin led to the search for derivatives which could treat a wider range of infections. The first real step forward was in the form of ampicillin. Ampicillin offered a broader spectrum of activity than either of the original agents and allowed doctors to treat a broader range of both Gram-positive and Gram-negative infections. Further developments led to amoxicillin, with improved duration of action.

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