Amoxicillin (INN) or amoxycillin (BAN) is a moderate-spectrum, bacteriolytic, β-lactam antibiotic used to treat bacterial infections caused by susceptible microorganisms. It is usually the drug of choice within the class because it is better absorbed, following oral administration, than other beta-lactam antibiotics. Amoxicillin is susceptible to degradation by β-lactamase-producing bacteria, and so may be given with clavulanic acid to decrease its susceptibility.

Mode of action
Amoxicillin acts by inhibiting the synthesis of bacterial cell walls. It inhibits cross-linkage between the linear peptidoglycan polymer chains that make up a major component of the cell wall of Gram-positive bacteria.

Microbiology

Amoxicillin is a moderate-spectrum antibiotic active against a wide range of Gram-positive, and a limited range of Gram-negative organisms. Some examples of susceptible and resistant organisms, from the Amoxil Approved Product Information (GSK, 2003), are listed below.

Susceptible Gram-positive organisms
— Streptococcus spp.
— penicillin-susceptible Streptococcus pneumoniae
— non β-lactamase-producing Staphylococcus spp.
— Enterococcus faecalis.

Susceptible Gram-negative organisms
Non-β-lactamase producing strains of the following bacteria:
— Haemophilus influenzae
— Neisseria gonorrhoeae
— Neisseria meningitidis
— Escherichia coli
— Proteus mirabilis
— Salmonella spp.

Resistant organisms
Penicillinase-producing organisms, particularly penicillinase-producing Staphylococcus spp. Penicillinase-producing N. gonorrhoeae and H. influenzae are also resistant.

All strains of Pseudomonas spp., Klebsiella spp., Enterobacter spp., indole-positive Proteus spp., Serratia marcescens, and Citrobacter spp. are resistant.

The incidence of β-lactamase-producing resistant organisms, including E. coli, appears to be increasing. Doctors may opt to prescribe co-amoxiclav, amoxicillin combined with β-lactamase inhibitor potassium clavulanate, instead of just amoxicillin to increase the efficacy.

Doubling the routinely given concentration (in pediatrics) of amoxicillin has been shown to eradicate intermediately resistant Streptococcus pneumoniae in selected infections.[1]

Formulations
Amoxicillin in trihydrate form is available as capsules, chewable and dispersable tablets plus syrup and paediatric suspension for oral use, and as the sodium salt for intravenous administration. It is one of the most common antibiotics issued to children, and the sweet-tasting liquid forms are helpful where the patient might find it difficult to take tablets or capsules. It has 3 ionizable groups.

Amoxicillin and clavulanic acid
To overcome the issue of β-lactamase production by resistant organisms, amoxicillin (in either trihydrate or sodium salt forms) may be combined with clavulanic acid, typically as the potassium salt. This combination has activity against a very broad array of Gram-positive, Gram-negative, and anaerobic organisms. It is not active against MRSA, P. aeruginosa, or C. difficile. It is available in oral preparations worldwide and also in the intravenous preparation in some countries. The British Approved Name for this formulation is co-amoxiclav, but it is commonly referred to in practice by proprietary names such as Amoksiklav, Augmentin, Clamoxyl, Augclac, Augmexx, and Yucla depending on country.

Side effects
Side effects are as those for other beta-lactam antibiotics. Side effects include nausea, vomiting, and easy fatigue. Loose bowel movements (diarrhea) also may occur.

The onset of an allergic reaction to amoxicillin can be very sudden and intense — emergency medical attention must be sought as quickly as possible. The initial onset of such a reaction often starts with a change in mental state; skin rash with intense itching (often beginning in fingertips and around groin area and rapidly spreading) and sensations of fever, nausea and vomiting. Any other symptoms that seem even remotely suspicious must be taken very seriously.

Non-allergic amoxicillin rash
Somewhere between 3% to 10% of children taking amoxicillin (or ampicillin) show a late-developing (>72 hours after beginning medication and having never taken penicillin-like medication previously) non-itchy rash, sometimes referred to as the «amoxicillin rash.» The rash is described as maculopapular or morbilliform (measles-like), and starts on the trunk and can spread from there. This rash is unlikely to be a true allergic reaction, and is not a contra-indication for future amoxicillin usage, nor should current regimen necessarily be stopped. However, as mentioned above, this common amoxicillin rash and a dangerous allergic reaction cannot easily be distinguished by inexperienced persons, and therefore a health professional should be consulted if a rash develops. (Pichichero, 2005; Schmitt 2005)