What is the equivalent dosage of dexamphetamine (Dextroamphetamine) when converting from Lisdexamphetamine (Vyvanse)?

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