Updated 28 June 2020 and September 2026

Child Antibiotic Dosing — Is It Child’s Play?

The point is that if antibiotic dosing in children is serious business, it can also be easy. Be mindful, but remain practical.

Use a reversion-to-the-mean approach that covers the large number of circumstances seen in most primary care settings. Because this is primary care and not, say, a university hospital clinic, most patients will, for all intents and purposes, be previously reasonably well and appropriate weight for age. But one look at the child will confirm that, if you’re ever in doubt.

A one-year-old child, for instance, will invariably be close to the 10 kg weight mark. Of course, some infants weigh more and some less (obvious at a glance from any percentile weight chart) — but generally speaking, 10 kg is the plumb-line to work from in one-year-olds.

Children’s antibiotic suspensions (most children’s oral drugs, in fact) come in concentrations designed so the dose conveniently falls within the relative proximity of 5 mL.

The dose should never be an order of magnitude higher or lower than that 5 mL guide. If calculations suggest 50 mL per dose or, conversely, 0.5 mL per dose for any oral formulation, chances are you’ve miscalculated. Time to recheck.

Should this child even be getting an antibiotic?

Before the dosing arithmetic: current Australian guidance (RACGP’s First Do No Harm, and RCH’s own acute otitis media guideline) is explicit that most cases of AOM in children resolve spontaneously, and antibiotics are not routinely recommended. First-line management for an otherwise well child over 6 months is adequate analgesia and a watchful-waiting approach, with review at 48–72 hours if symptoms haven’t settled.

Antibiotics are reasonable to prescribe upfront when the child:

• is under 6 months old
• is systemically unwell
• has bilateral AOM and is under 2 years old
• has otorrhoea from tympanic membrane perforation
• is from a population at higher risk of suppurative complications (e.g. Aboriginal or Torres Strait Islander children, particularly in remote communities)
• is immunocompromised, has a cochlear implant, or has only one hearing ear
• has recurrent AOM or craniofacial abnormalities

Worth having this framing in mind before the “which dose” question — for many presentations, the right answer is analgesia and a review appointment, not a script.

When you do prescribe: amoxicillin remains first-line

In Australia, amoxicillin comes in three suspension strengths for convenient childhood dosing — each landing somewhere in the vicinity of 5 mL, or a traditional teaspoon, including for neonates:

• 100 mg/mL [20 mL bottle with dropper]: neonate
• 125 mg/5 mL [100 mL bottle]: infant/toddler
• 250 mg/5 mL [100 mL bottle]: pre-school/school-aged

Current Australian Therapeutic Guidelines and RCH recommend amoxicillin dosed at 45–50 mg/kg/day, in 2–3 divided doses, for AOM. (This is a genuine and longstanding difference from current US AAP guidance, which recommends higher-dose 80–90 mg/kg/day amoxicillin — reflecting Australia’s comparatively lower rates of resistant pneumococcus. If you’re cross-checking against a US reference, don’t be surprised by the gap; it isn’t an error in either guideline.)

At twice-daily dosing (50 mg/kg/day), that’s conveniently 25 mg/kg per dose — multiply the child’s weight by 25 to get the per-dose amount in mg. A 10 kg child needs 250 mg per dose, which is 5 mL of the 250 mg/5 mL concentration — a clean, round number that keeps the “somewhere near 5 mL” rule intact.

• 10 kg child: 250 mg, or 5 mL, BD (250/5)
• 12 kg child: 300 mg, or 6 mL, BD (250/5)
• 15 kg child: 375 mg, or about 7.5 mL, BD (250/5)

At three-times-daily dosing (45 mg/kg/day), that’s 15 mg/kg per dose — multiply the child’s weight by 15. A 10 kg child needs 150 mg per dose, which is 6 mL of the 125 mg/5 mL concentration, or 3 mL of the 250 mg/5 mL concentration.

Worked example

Presentation: A one-year-old child has an inflamed, bulging right tympanic membrane. They’re off their food, febrile, and have vomited. No history of allergies.

Reasoning: Unilateral AOM in a child this age wouldn’t automatically mandate antibiotics on its own — but being off food, febrile, and vomiting raises the question of whether this child is more systemically unwell than a straightforward, well-looking case of AOM. That’s a judgement call at the bedside (how unwell do they actually look, how hydrated are they, is there anything pointing to a different or more serious diagnosis) rather than something the history alone settles. If you conclude they warrant antibiotics rather than a watchful-waiting review, the dosing follows directly:

Antibiotic treatment (if prescribed): Amoxicillin 250/5, 5 mL BD PO × 5 days.

Question: What antibiotic and dose would you give this child, if you’ve decided antibiotics are warranted?

Answer: If no penicillin allergy, 5 mL BD of the 250/5 concentration of amoxicillin.

Look at the child and confirm they’re about 10 kg. Ask the parents for confirmation if you prefer. Ask about allergies. Then dose 250/5 amoxicillin at 5 mL twice a day.

One-year-old, middle ear infection, antibiotics indicated: 5 mL amoxicillin (250/5) twice a day.

If the child is 9.2 kg or 10.6 kg, you can quickly and safely treat with the same 5 mL BD of 250/5 amoxicillin — the dosing doesn’t need to track weight to the decimal point at this level of precision.

For more severe, recurrent, or systemically unwell presentations, dose escalation may be warranted — check local antimicrobial guidelines for the specific regimen rather than empirically doubling.

Why not just use a flat age- or weight-based rule?

Traditional child dosages for amoxicillin — historically cited around 20 mg/kg — were based on 1970s trial data. But children are not small adults, pharmacokinetically:

• composition of intestinal fluid and gut permeability varies in childhood
• gastric pH decreases during infancy, reaching adult levels (pH ~2) around age 2
• volume of distribution changes throughout childhood
• infants have a higher percentage of extracellular water than older children
• infants have lower concentrations of circulating plasma proteins, and consequently lower drug-protein binding
• cytochrome P450 enzymatic activity changes over the first few months of life — children may even exceed adult levels by around age 2
• hepatic glucuronidation is thought to take at least 3 years to reach full activity
• GFR doesn’t reach adult levels until around age 2¹

Dosage regimens based entirely on age, or extrapolated from adult data, are often inaccurate and may lead to adverse effects, toxicity, or lack of clinical effect.¹ When dosing children, refer to a paediatric reference source rather than adult-derived approximations.

Similar logic applies to other antibiotics

• Erythromycin, for penicillin-allergic patients or atypical cover, is often dosed around 30–40 mg/kg/day (avoid in neonates — increased risk of pyloric stenosis — and use cautiously given its arrhythmogenic potential). Worth noting: RCH’s own current otitis media guidance doesn’t actually reach for erythromycin as the penicillin-allergy fallback for AOM specifically — it steps up to amoxicillin-clavulanate instead if response is inadequate.
• Cephalexin, commonly used for UTI and skin/soft tissue infections, is dosed at 25–50 mg/kg/day depending on indication and severity (confirmed against RCH’s own guideline).
• Metronidazole, for intestinal infestations such as giardiasis, is dosed at 30 mg/kg/day (confirmed against both RCH and Quebec paediatric dosing references).
• Phenoxymethylpenicillin (oral penicillin) for tonsillitis is dosed by RCH as a flat age-band rather than mg/kg: 250 mg BD under 10 years, 500 mg BD from 10 years, for 10 days.

As always: check current local or national guidelines for the specific indication rather than relying on a single memorised number across all uses of a drug — the “convenient round number” habit is a starting scaffold for busy clinical moments, not a substitute for checking when the stakes or complexity are higher.

Annotations

• 125/5 means 25 mg of active drug per 1 mL of suspension
• 250/5 means 50 mg of active drug per 1 mL of suspension

References

1. O’Hara, Kate. “Paediatric pharmacokinetics and drug doses.” Aust Prescr 2016;39:208–10. http://dx.doi.org/10.18773/austprescr.2016.071
2. RACGP. First do no harm: Antibiotic use for acute otitis media in children. 2025.
3. The Royal Children’s Hospital Melbourne. Clinical Practice Guidelines: Antibiotics and Acute otitis media. (Also linked as downloads on this post.)
4. Centre d’information du médicament du Québec (CDM). Dosage Guidelines for Commonly Used Antibiotics in Children. (Also linked as a download on this post.)
5. Australian Prescriber. “Managing otitis media: an evidence-based approach.”
6. American Academy of Pediatrics. “The Diagnosis and Management of Acute Otitis Media.” Pediatrics. 2013;131(3):e964–999. (Cited for contrast with US high-dose practice — see note above.)

Because of all these variable parameters, childhood age classes are often divided commensurately into:

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Downloads

CdM-Antibio1-DosageGuidelines-Children-en

Clinical Practice Guidelines _ Antibiotics– RCH

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References

O’Hara, Kate. “Paediatric pharmacokinetics and drug doses.” Aust Prescr 2016;39:208–10. Available at http://dx.doi.org/10.18773/austprescr.2016.071. Accessed 28 June 2020.