Paediatric Amoxicillin Dose and Leftover Antibiotic
A 4-year-old boy, weight 16 kg, has acute otitis media. The prescription is amoxicillin 90 mg/kg/day divided twice daily for 10 days. The pharmacy stocks amoxicillin 250 mg/5 mL in 100 mL bottles. The mother has an opened bottle of amoxicillin 400 mg/5 mL from the child's sibling, dispensed 3 months ago and kept in the refrigerator; she asks whether she can use it to avoid buying more. The child has no penicillin allergy, takes no other medicines, and has had diarrhea with antibiotics before. His temperature is 38.2°C, and he is drinking but has vomited once. The prescription is otherwise complete. The pharmacy has oral dosing syringes but no new 400 mg/5 mL stock. The mother wants to start treatment tonight.
1. Calculate the dose and volume using the stocked 250 mg/5 mL suspension, and determine how many bottles are needed.
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90 mg/kg/day × 16 kg = 1,440 mg/day; divided twice daily = 720 mg per dose. With 250 mg/5 mL (50 mg/mL), 720 ÷ 50 = 14.4 mL per dose. For 10 days: 14.4 mL × 2 × 10 = 288 mL; 100 mL bottles provide 100 mL each, so dispense three bottles (300 mL). The opened 400 mg/5 mL bottle from the sibling should not be used: it was opened 3 months ago, is beyond stability, may be contaminated, and has a different concentration. If new 400 mg/5 mL stock were available, the dose would be 720 ÷ 80 = 9 mL twice daily and 180 mL total, but the prescription should not be changed without prescriber authorization. Measure 14.4 mL accurately with an oral syringe.
2. What counselling points should the pharmacist give the mother?
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Shake the bottle well before each dose and use the oral dosing syringe, not a kitchen spoon. Refrigerate the suspension and discard any remaining product after 14 days or by the manufacturer's beyond-use date, whichever is earlier. Complete all 10 days even if the child improves. Give with food if stomach upset occurs; avoid mixing the dose into a full bottle of milk or juice because the child may not finish it. Counsel about rash, vomiting, severe diarrhea, or poor feeding. Do not use leftover antibiotics for future illness or share with siblings. Write the dose and times on the label and use teach-back.
3. The mother reports the child vomited 20 minutes after the first dose. What is your assessment and advice?
Read the worked answer
Vomiting within about 20-30 minutes means the dose may not have been absorbed. If the child is otherwise well and this is an isolated episode, give one repeat dose with food or milk and observe. If vomiting recurs, the child cannot keep fluids down, or there are signs of dehydration or severe illness, advise urgent medical review rather than repeated dosing. Do not double the next scheduled dose. Document the advice and follow up to confirm the child is tolerating the antibiotic.