Lisdexamfetamine to Dexamphetamine Conversion
Direct Conversion Formula
When converting from lisdexamfetamine (Vyvanse) to dexamphetamine (Dextroamphetamine), use a conversion ratio of approximately 0.3, meaning 100 mg of lisdexamfetamine yields approximately 30 mg of active dexamphetamine. 1, 2
Conversion Calculation
- For every 100 mg of lisdexamfetamine, expect approximately 30 mg of active dexamphetamine to be released systemically 2
- The conversion is based on the molecular weight difference and the fact that lisdexamfetamine is a prodrug that must be hydrolyzed to release active dexamphetamine 1, 2
- Example: A patient on lisdexamfetamine 70 mg would convert to approximately 21 mg of dexamphetamine equivalents 2
Practical Dosing After Conversion
Start with 70-75% of the calculated equivalent dose when switching from lisdexamfetamine to immediate-release dexamphetamine, then titrate by 2.5-5 mg increments weekly based on response. 3, 4
Specific Conversion Examples:
- Lisdexamfetamine 30 mg → Start dexamphetamine 7.5 mg total daily (split as 2.5 mg TID or 5 mg BID) 3, 4
- Lisdexamfetamine 50 mg → Start dexamphetamine 10-12.5 mg total daily (split as 5 mg BID or 5 mg TID) 3, 4
- Lisdexamfetamine 70 mg → Start dexamphetamine 15 mg total daily (split as 5 mg TID or 7.5 mg BID) 3, 4
Dosing Schedule Options
- For twice-daily dosing: Give the total daily dose split evenly in morning and early afternoon (before 2 PM) 3, 4
- For three-times-daily dosing: Split the total daily dose into morning, midday, and early afternoon doses for more consistent coverage 3, 4
- Consider dexamphetamine Spansule (extended-release) once daily in the morning if available, using the calculated total daily dose 3, 4
Critical Pharmacokinetic Differences
- Lisdexamfetamine has a delayed Tmax of approximately 3.5 hours for active dexamphetamine versus 1 hour for immediate-release dexamphetamine 2, 5
- The lag time for lisdexamfetamine is approximately 1 hour longer than immediate-release dexamphetamine 5
- Immediate-release dexamphetamine will have a faster onset (within 30-60 minutes) and shorter duration (4-6 hours per dose) compared to lisdexamfetamine's 13-14 hour duration 6, 5
- Peak plasma concentrations occur earlier with immediate-release dexamphetamine, which may increase side effects initially 5
Titration and Monitoring Protocol
- Allow at least one week between dose adjustments to properly evaluate response 3, 4
- Titrate in 2.5-5 mg increments based on symptom control and tolerability 3, 4
- Maximum recommended daily dose is 40-50 mg for dexamphetamine 3, 4, 7
- Monitor blood pressure and pulse at baseline and with each dose adjustment 4
- Weigh patient at each visit to monitor for appetite suppression 4
Common Pitfalls to Avoid
- Do not give late afternoon or evening doses—last dose should be before 2-3 PM to minimize sleep disturbances 4
- Do not exceed maximum recommended doses (40-50 mg/day) if inadequate response occurs—instead reassess diagnosis and consider switching to methylphenidate or non-stimulant alternatives 3, 4, 7
- Patients may initially report decreased duration of effect compared to lisdexamfetamine, which is expected given the shorter half-life of immediate-release formulations 6, 5
- Approximately 70% of patients respond to a single stimulant type, but nearly 90% respond if both amphetamine and methylphenidate are tried 3, 4
Special Considerations for Immediate-Release Formulations
- The faster onset and offset of immediate-release dexamphetamine may cause more noticeable "peaks and valleys" in symptom control compared to lisdexamfetamine 6, 5
- Some patients may require a third afternoon dose to maintain coverage through homework or work hours 3, 4
- For children weighing less than 25 kg, single doses should generally not exceed 15 mg 3, 7