Oxybutynin Dosing for 18 kg Child
For a child weighing 18 kg, the recommended dose of oxybutynin is 3.6 mg three times daily (0.2 mg/kg per dose), for a total daily dose of 10.8 mg. 1, 2
Dosing Calculation and Rationale
- The standard pediatric dose is 0.2 mg/kg three times daily, which is specifically recommended for children with neurogenic bladder and detrusor overactivity 1, 2
- For an 18 kg child: 18 kg × 0.2 mg/kg = 3.6 mg per dose
- Total daily dose: 10.8 mg (3.6 mg × 3 times daily)
Evidence Supporting This Dose
- The FDA label confirms that pediatric patients with detrusor overactivity have been successfully treated with total daily doses ranging from 7.5 mg to 15 mg (0.22 to 0.53 mg/kg), with most patients (86.9%) taking between 10-15 mg daily 3
- The Journal of Urology specifically recommends starting at 0.2 mg/kg three times daily for children with hostile bladder on urodynamic evaluation 2
- Long-term studies in children have shown this dosing to be both effective and safe, with no different side effect profile compared to adults 4
Formulation Considerations
- Oxybutynin tablets come in 5 mg scored tablets, making precise dosing challenging for 3.6 mg 3
- Consider using oxybutynin syrup/solution if available, which allows for more accurate weight-based dosing 3
- If only tablets are available, you may need to round to the nearest practical dose (either 2.5 mg or 5 mg per dose) and adjust based on clinical response
Important Prescribing Caveats
- Behavioral interventions should be attempted first before initiating oxybutynin therapy 1, 5
- Exclude or treat constipation before starting anticholinergic therapy to minimize gastrointestinal side effects 1, 5
- Assess post-void residual before initiating therapy if the child is at higher risk of urinary retention 1, 5
- Reassess after 4-8 weeks using validated symptom questionnaires and voiding diaries to evaluate treatment efficacy 5
Absolute Contraindications
Expected Side Effects
- The most common side effects are dry mouth and constipation (gastrointestinal symptoms) 4
- Urological signs such as dysuria and urinary retention may occur but are less common 4
- Overall, 26-30% of patients report side effects, with approximately 10% discontinuing due to severity 4, 6
Dose Titration Strategy
- If the standard dose (0.2 mg/kg TID) is ineffective after 4-8 weeks and well-tolerated, consider dose escalation 5
- Research supports escalating in steps of 0.2 mg/kg per day up to 0.9 mg/kg per day for refractory cases 7
- For this 18 kg child, escalation could go up to approximately 16 mg total daily dose if needed, though this should be done cautiously and with close monitoring 7