Amoxicillin has been around since 1972. It's prescribed billions of times every year. Most people have taken it at least once in their lives — for a throat infection, a chest infection, a dental procedure, or an ear problem. And yet most people have no idea how their dose was actually determined, or why taking the wrong amount is a bigger problem than it sounds.
This guide won't replace your doctor. But it will help you understand what you've been prescribed — and why.
What Amoxicillin Actually Does
Amoxicillin is a penicillin-type antibiotic. It works by attacking the cell walls of certain bacteria — specifically, it blocks the proteins bacteria need to build and maintain their outer walls. Without an intact cell wall, the bacteria can't survive.
It's effective against a wide range of bacterial infections including strep throat, ear infections, urinary tract infections, pneumonia, skin infections, and H. pylori stomach infections. It does absolutely nothing against viral infections — including the common cold, flu, and COVID-19. This is an important distinction because amoxicillin is frequently requested for viral illnesses where it provides zero benefit and contributes to antibiotic resistance.
Standard Adult Dosage — The General Framework
For most common infections in adults, the typical amoxicillin dosage falls within these ranges:
- Mild to moderate infections — 250mg to 500mg every 8 hours, or 500mg to 875mg every 12 hours
- Severe infections — 875mg every 12 hours or 500mg every 8 hours
- Dental infections and procedures — Often a single 2000mg (2g) dose taken one hour before the procedure
- H. pylori eradication — Usually 1000mg twice daily, always combined with other medications
These are general frameworks — not prescriptions. Your doctor's specific instructions always take priority over general guidelines, because your dose depends on factors that a general chart cannot account for.
Paediatric Dosage — Why Children Are Different
Children are not small adults. Their dosage is calculated based on body weight — typically 25mg to 45mg per kilogram of body weight per day, divided into doses every 8 or 12 hours. The exact amount depends on the type and severity of infection.
This weight-based calculation is why it's particularly important not to guess or estimate a child's dose. Under-dosing risks treatment failure. Over-dosing raises the risk of side effects. A proper calculation using current weight is always necessary.
- Ear, nose, and throat infections (mild/moderate): 25mg/kg/day in divided doses
- More severe infections or resistant strains: 40–45mg/kg/day in divided doses
- Maximum paediatric dose generally should not exceed standard adult doses
Kidney Function Changes Everything
Amoxicillin is cleared from the body primarily through the kidneys. If kidney function is reduced — due to age, chronic kidney disease, or other conditions — the drug stays in the body longer than it should. This means standard doses can accumulate to levels that cause toxicity.
Doctors adjust amoxicillin dosage based on a measurement called GFR (Glomerular Filtration Rate), which reflects how well the kidneys are filtering. Patients with significantly reduced kidney function may receive lower doses, less frequent dosing intervals, or an alternative antibiotic entirely. This is one of the main reasons self-prescribing antibiotics without a proper assessment is genuinely risky — not just legally, but medically.
The Full Course Rule — Why You Cannot Stop Early
You feel better after three days. The infection seems gone. The remaining seven days of tablets sit on your nightstand and you think — do I really need to finish these?
Yes. Always.
Feeling better does not mean the bacteria are completely eliminated. Stopping antibiotics early leaves a surviving population of bacteria that are, by definition, the ones that were slightly more resistant to the drug. These bacteria multiply. The infection can return — this time harder to treat. This is one of the primary mechanisms behind antibiotic resistance, which is now one of the most significant public health threats globally.
Finish the course. Every time. Even when you feel completely fine.
Side Effects Worth Knowing About
Amoxicillin is generally well tolerated, but side effects do occur. The most common ones are gastrointestinal — nausea, diarrhoea, and stomach discomfort. Taking amoxicillin with food reduces these significantly for most people.
More serious reactions to be aware of:
- Allergic reaction — Rash, hives, or itching can indicate an allergy. Stop the medication and contact a doctor immediately.
- Severe allergic reaction (anaphylaxis) — Difficulty breathing, swelling of face or throat, rapid heartbeat. This is a medical emergency. Call emergency services immediately.
- C. difficile colitis — Severe, persistent diarrhoea during or after antibiotic use can indicate a serious secondary infection. Always report this to a doctor.
If you have a known penicillin allergy, amoxicillin must be avoided entirely. Always inform your doctor and pharmacist of any known drug allergies before starting any antibiotic.
Calculate the Right Dose for Your Weight
Need to calculate a weight-based amoxicillin dose — particularly for a child? Our free Amoxicillin Dosage Calculator works out the correct dose range based on weight, age group, and infection type. Always use the result as a reference alongside your doctor's advice — not as a replacement for it.