Acetaminophen and Ibuprofen Dosing for a 9-Year-Old, 90 lbs
For this 9-year-old child weighing 90 lbs (41 kg), give acetaminophen (Tylenol) 600 mg every 4-6 hours as needed (maximum 2400 mg/day) and ibuprofen (Motrin) 400 mg every 6 hours as needed (maximum 1600 mg/day).
Acetaminophen (Tylenol) Dosing
The standard pediatric dosing for acetaminophen is 10-15 mg/kg per dose 1. For a 41 kg child:
- Calculated dose: 410-615 mg per dose
- Practical rounded dose: 600 mg every 4-6 hours
- Maximum daily dose: 2400 mg (four 600 mg doses in 24 hours)
This dosing is based on weight-based calculations that keep each dose within the recommended therapeutic range while being practical for home administration 1.
Ibuprofen (Motrin) Dosing
The standard pediatric dosing for ibuprofen is 10 mg/kg per dose 2, 3. For a 41 kg child:
- Calculated dose: 410 mg per dose
- Practical rounded dose: 400 mg every 6 hours
- Maximum daily dose: 1600 mg (four 400 mg doses in 24 hours)
At this weight, the child falls into the adult dosing category per FDA labeling, which recommends 200-400 mg per dose for children 12 years and older 4. However, weight-based dosing of 10 mg/kg is more appropriate for a 9-year-old, yielding 400 mg per dose.
Comparative Efficacy
Both medications have similar efficacy for pain relief 2, 5. For fever reduction specifically, ibuprofen 10 mg/kg demonstrates modestly superior antipyretic effects compared to acetaminophen 10-15 mg/kg at 2,4, and 6 hours post-treatment 2, 3. The weighted effect sizes favor ibuprofen, particularly at the 4-hour mark (effect size 0.81) 2.
Combined or Alternating Therapy
If single-agent therapy is insufficient, combined or alternating therapy may provide superior fever control 6, 7:
- Combined therapy: Give both medications together at the start
- Alternating therapy: Give ibuprofen first, then acetaminophen 3 hours later
Studies show that 97-100% of children receiving combined or alternating therapy were afebrile at hours 4-6, compared to only 50-70% receiving ibuprofen alone 6. Network meta-analysis confirms that combined therapy (OR 0.19) and alternating therapy (OR 0.20) are superior to acetaminophen alone for achieving fever resolution at 4 hours 7.
Safety Considerations
Both medications are well-tolerated at recommended doses with similar safety profiles 2, 5. Key precautions:
- Do not exceed maximum daily doses
- Space doses appropriately (acetaminophen every 4-6 hours; ibuprofen every 6 hours)
- Avoid ibuprofen in children with aspirin-induced asthma or active bleeding disorders
- Use measuring devices (oral syringes) rather than household spoons to ensure accuracy
Practical Formulation Guidance
For a 9-year-old, tablets or capsules are typically appropriate rather than liquid suspensions. Standard over-the-counter formulations include:
- Acetaminophen: 325 mg or 500 mg tablets (give two 325 mg tablets for 650 mg dose, or round to 600 mg with appropriate formulation)
- Ibuprofen: 200 mg tablets (give two tablets for 400 mg dose)
Avoid the infant concentration of ibuprofen (200 mg/5 mL) as it has significantly higher dosing error rates compared to children's concentration (100 mg/5 mL) 8.