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Amoxicillin is a semisynthetic (synthesized from naturally occurring substance) aminopenicillin antibiotic structurally related to ampicillin. It is a moderate-spectrum antibiotic active against a wide range of Gram-positive, and a limited range of Gram-negative organisms.

Amoxicillin was discovered by scientists at Beecham Research Laboratories in 1972. In the US it is marketed by GlaxoSmithKline (the inheritor company) under the original trade name Amoxil.

The narrow spectrum of antimicrobal activity of the penicillins, led to the search for derivatives of penicillin which could treat a wider range of infections. The first important step forward was the development of ampicillin. Ampicillin had a broader spectrum of activity than either of the original penicillins 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.

Amoxicillin is indicated for the treatment of the following infections due to susceptible strains of sensitive organisms:

• Ear, nose, and throat infections - otitis media (middle ear infection), pharyngitis (sore throat), sinusitis (sinus infection), tonsillitis, and tonsillopharyngitis.
• Genitourinary tract infections - infections of the genital organs (vagina, uterus, fallopian tubes/ovaries) and the urinary tract (kidneys, ureters, bladder and urethra)
• Skin and skin structure infections - cellulitis (infection of the dermis and subcutaneous tissue), erysipelas (superficial form of cellulitis), folliculitis (inflammation of the hair follicles, if the infection of the follicle is deeper and involves more follicles, it moves into the furuncle and carbuncle), furuncles, carbuncles, abscesses, impetigo (large vessicles or honey-crusted sores), infected ulcers and infected burns and other.
• Lower respiratory tract infections - tracheobronchitis, bronchitis, pneumonia.
• Sexually transmitted diseases - gonorrhea, acute uncomplicated (ano-genital and urethral infections)
• Helicobacter pylori infection eradication to reduce the risk of duodenal ulcer recurrence

Off-label and investigational uses of amoxicillin include:

• Chlamydial infections. Chlamydia trachomatis is an obligate intracellular bacteria, considered to be the most commonly sexually transmitted organism. Amoxicillin is a recommended treatment of Chlamydia infection during pregnancy by Centers for Disease Control and Prevention2, 8. However, it is not the best antibiotic for eradication of Chlamydia in other categories of people.
• Postexposure prophylaxis for anthrax. Amoxicillin has not been approved by the FDA to use when treating people who have been exposed to anthrax. However, if test results show that the anthrax bacteria can be killed by penicillin antibiotics, amoxicillin is recommended5 to prevent the development of anthrax disease in people who have been exposed to anthrax, when other antibiotics (ciprofloxacin and doxycycline) may be contraindicated.
• Endocarditis (inflammation of the inner lining of the heart). Bacterial endocarditis is a relatively uncommon, life-threatening infection of the inner lining of the heart. The most common antibiotic used to prevent bacterial endocarditis is Amoxicillin. The recommended standard prophylactic regimen is a single dose of oral amoxicillin (2.0 g in adults and 50 mg per kg in children)6.
• Typhoid Fever (enteric fever). Typhoid fever is potentially fatal if untreated. Amoxicillin is very effective and reduces the risk of the carrier state. The antibiotic is usually given for 14 days.
• Lyme disease. Lyme disease is caused by the spirochetal bacterium Borrelia burgdorferi, and is transmitted to people by tick bites. A 20 to 30 days course of amoxicillin is generally effective in early disease7. Amoxicillin can speed the healing of the erythema migrans rash and usually prevent following symptoms such as arthritis or neurological problems.

• Advantages:
• Better absorbed from the gastrointestinal tract than other penicillins (penicillin V, ampicillin). Amoxicillin provides higher and more sustained blood antibiotic levels.
• Amoxicillin is the first choice antibiotic for middle ear infection (Otitis media)3.
• Excellent tissue penetration. Amoxicillin diffuses readily into most body tissues and fluids, with the exception of brain and spinal fluid.
• It appears to be safe for use in pregnancy4 and is widely used in the treatment of various infections in pregnant women.
• Safe for children.
• Inexpensive and available in generic formulation.
• Long-established track record in scientific studies and in practice. Amoxicillin has been extensively researched for more than 40 years.
• Disadvantages:
• Allergy to amoxicillin (Penicillin hypersensitivity reaction). The penicillin allergy results from the formation of an allergen when the beta-lactam ring reacts with a terminal amine on a lysine residue in a polypeptide. Every time a person is exposed to this allergen, their immune system will sometimes cause an hypersensitivity (anaphylactic) reaction.
• More likely to cause diarrhea than other antibiotics 1.
• Frequent dosing regimen.
• Test interactions. May interfere with urinary glucose tests using cupric sulfate (Benedict's solution, Clinitest).
• Not effective against beta-lactamase-producing bacteria.
• Tooth enamel defects. According to the recent study in the Archives of Pediatrics and Adolescent Medicine9 the use of the amoxicillin during infancy appears to be linked to tooth enamel defects in permanent teeth. However, further research is needed.

Penicillins are bactericidal antibiotics (they kill the bacteria). Amoxicillin works by inhibiting the synthesis of bacterial cell walls.

Like other penicillins, amoxicillin inhibits the third and final stage of bacterial cell wall synthesis by preferentially binding to specific penicillin-binding proteins (PBPs) that are located inside the bacterial cell wall.

The half-life of amoxicillin is about 61.3 minutes but in the absence of renal function it is 7 to 10 hours. The half-life may be longer in neonates and the elderly. In general, it takes 5-6 hours for amoxicillin to leave the system.

Strep throat (Streptococcal pharyngitis) is a form of Group A streptococcal infection that affects the pharynx. It is most common in children between the ages of 5 and 15, but it affects people of all ages. The bacteria that cause strep throat respond quickly to antibiotics, and treatment stops the infection from spreading to other people. Amoxicillin is one of the most frequently prescribed antibiotics for treating strep throat.

The cause of strep throat is bacteria known as Streptococcus pyogenes, or group A beta-hemolytic streptococcus.

Dosage recommendations vary based on the severity of the infection:

• For adults for mild or moderate infections, the usual dosage is 250 mg every 8 hours, or 500 mg every 12 hours.
• For adults for severe infections, the usual dosage is 500 mg every 8 hours, or 875 mg every 12 hours.
• With most medicines for children, the dosage varies with the child's weight.
  For most infections, children older than 3 months but less than 40 kg are treated with 25 mg/kg/day in divided doses every 12 hours, 20 mg/kg/day in divided doses every 8 hours, 40 mg/kg/day in divided doses every 8 hours or 45 mg/kg/day in divided doses every 12 hours depending on type and severity of the infection.

Most scientific studies of amoxicillin effectiveness have used a regime of three times a day for ten days. While this remains the standard, some studies have shown advantages of three, five, or seven day courses.

• Amoxicillin (Amoxil) versus Other Medications

• 1. Arguedas A, Emparanza P, Schwartz RH, Soley C, Guevara S, de Caprariis PJ, Espinoza G. A randomized, multicenter, double blind, double dummy trial of single dose azithromycin versus high dose amoxicillin for treatment of uncomplicated acute otitis media. Pediatr Infect Dis J. 2005 Feb;24(2):153-61. PubMed
• 2. Kacmar J, Cheh E, Montagno A, Peipert JF. A randomized trial of azithromycin versus amoxicillin for the treatment of Chlamydia trachomatis in pregnancy. Infect Dis Obstet Gynecol. 2001;9(4):197-202. PubMed
• 3. Quach C, Collet JP, LeLorier J. Effectiveness of amoxicillin, azithromycin, cefprozil and clarithromycin in the treatment of acute otitis media in children: a population-based study. Pharmacoepidemiol Drug Saf. 2005 Mar;14(3):163-70. PubMed
• 4. Jepsen P, Skriver MV, Floyd A, Lipworth L, Schnheyder HC, Srensen HT. A population-based study of maternal use of amoxicillin and pregnancy outcome in Denmark. Br J Clin Pharmacol. 2003 Feb;55(2):216-21. PubMed
• 5. Commentary on Non-Labeled Dosing of Oral Amoxicillin in Adults and Pediatrics for Post-Exposure Inhalational Anthrax. FDA Center for Drug Evaluation and Research
• 6. Kathryn A. Taubert, Adnan S. Dajani. Preventing Bacterial Endocarditis: American Heart Association Guidelines. Vol. 57/No. 3 (February 1, 1998) American Academy of Family Physicians.
• 7. Eppes SC, Childs JA. Comparative study of cefuroxime axetil versus amoxicillin in children with early Lyme disease. Pediatrics. 2002 Jun;109(6):1173-7. PubMed
• 8. Sexually transmitted diseases treatment guidelines 2002. Centers for Disease Control and Prevention.
• 9. 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. PubMed