Amoxicillin

BRAND NAMES 

Common International Brand Names and Common Indian Brand Names

• Amoxil

• Moxatag (extended-release)

• Trimox

• Moxilin

• Mox (Sun Pharma)

• Novamox (Cipla)

• Wymox (Abbott) 

MECHANISM OF ACTION 

Amoxicillin killed bacteria by interfering with the construction of their cell walls. It bound to penicillin-binding proteins (PBPs) and inhibited peptidoglycan synthesis. The resulting loss of wall strength caused bacterial cells to rupture and die.

Its antibacterial effect depended chiefly on the duration for which drug concentrations exceeded the organism’s minimum inhibitory concentration (MIC). Resistance arose when β-lactamase enzymes destroyed the drug or when modified PBPs reduced its binding. 

PHARMACOKINETICS 

Absorption

Amoxicillin remained stable in gastric acid and was rapidly absorbed when taken orally. Blood levels generally reached their peak within 1–2 hours. Oral bioavailability was approximately 70%, and food did not substantially reduce absorption 

Distribution

After absorption, amoxicillin reached a wide range of tissues and body fluids. About 20% was bound to serum proteins. Entry into cerebrospinal fluid was limited when the meninges were not inflamed. It also passed across the placenta and entered breast milk in small amounts. 

Metabolism

Amoxicillin underwent limited metabolic conversion. Some was transformed into inactive penicilloic acid, which appeared in urine in quantities corresponding to approximately 10–25% of the initial dose. 

Elimination

The kidneys were the principal route of removal. Around 60% of an oral dose left the body unchanged in urine within 6–8 hours. In individuals with normal renal function, the elimination half-life was approximately one hour. Impaired kidney function slowed clearance and could require an adjusted dose, while probenecid delayed excretion 

PHARMACODYNAMICS 

Amoxicillin belonged to the aminopenicillin group of β-lactam antibiotics. It attached to penicillin-binding proteins and disrupted bacterial cell-wall formation, leading to cell breakdown and death. Its bactericidal activity was time-dependent and relied mainly on how long its concentration remained above the minimum inhibitory concentration (MIC). Bacterial resistance commonly arose through β-lactamase production or changes in penicillin-binding proteins.

ADMINISTRATION 

Amoxicillin were swallowed whole with water. The suspension was shaken before each dose and measured using an oral syringe or medicine spoon. Doses were spaced evenly, and treatment was continued for the prescribed duration. A missed dose was taken when remembered unless the next dose was nearly due; doses were not doubled.

DOSAGE AND STRENGTH 

The following doses represented general prescribing-label examples rather than an individual prescription. 

• Tablets: 500 mg and 875 mg.

• Capsules: 250 mg and 500 mg.

• Oral suspension: 125, 200, 250, or 400 mg per 5 mL.

The dosage recommendations were provided as clinical guidance for qualified healthcare professionals. Treatment was tailored to the indication, the patient’s age and body weight, kidney and liver function, existing medical conditions, concurrent medications, and response to therapy. The complete prescribing information was reviewed before treatment was started or the dosage was adjusted.

DRUG INTERACTIONS 

Probenecid reduces the renal elimination of amoxicillin, resulting in higher and more prolonged blood concentrations. Concurrent use of allopurinol increases the likelihood of a skin rash. Amoxicillin may increase the international normalised ratio (INR) and bleeding risk in patients taking warfarin or similar anticoagulants, making careful monitoring necessary. It may also reduce the elimination of methotrexate, increasing the risk of methotrexate toxicity.

FOOD INTERACTIONS

Amoxicillin could generally be taken with or without food, although taking it with a meal or snack helped reduce nausea. Product-specific instructions still needed to be followed. No particular foods, including milk or dairy products, required avoidance. Alcohol had no specific interaction with amoxicillin but could worsen nausea or dehydration.

CONTRAINDICATIONS 

It was also avoided in patients with infectious mononucleosis because of the increased likelihood of a widespread rash. This was a precaution rather than the main formal contraindication. Severe kidney impairment required an adjusted dose but did not automatically prevent treatment.

SIDE EFFECTS 

Common adverse effects included nausea, vomiting, diarrhoea and skin rash. Oral or vaginal thrush also occurred in some patients.

Serious reactions included anaphylaxis, blistering or peeling skin, and Clostridioides difficile infection, which caused severe watery or bloody diarrhoea and could develop after treatment had ended. Breathing difficulty, swelling of the tongue or throat, collapse, or a blistering rash required immediate discontinuation and emergency medical care.

OVERDOSE 

An overdose could cause abdominal pain, vomiting, blood in the urine, difficulty urinating or reduced urine output. Large overdoses could lead to urinary crystal formation and acute kidney injury.

Management involved supportive care, medically supervised fluid administration and monitoring of kidney function. No specific antidote was available, although haemodialysis could remove amoxicillin when necessary. Suspected overdose required prompt advice from a poison centre or urgent medical service. Inducing vomiting was discouraged, and the medicine packaging needed to be kept available.

TOXICITY 

Toxicity could result from excessive dosing, drug accumulation or an adverse reaction at a prescribed dose. In addition to the kidney complications described above, reported effects included seizures, confusion, liver injury and abnormalities in blood-cell counts. Reduced kidney function increased the risk of amoxicillin accumulation.