Dose Conversion from Vyvanse to Adderall
When switching from 40 mg of Vyvanse to Adderall in a patient in their 20s, start with 10-15 mg of Adderall (not 20 mg), as lisdexamfetamine is converted to dextroamphetamine at approximately a 1:0.3 ratio, making 40 mg of Vyvanse roughly equivalent to 12 mg of dextroamphetamine base. 1, 2
Understanding the Conversion Ratio
Lisdexamfetamine (Vyvanse) is a prodrug that is converted to dextroamphetamine after ingestion, with the conversion occurring through enzymatic hydrolysis 1, 3
The molecular weight difference means that 40 mg of lisdexamfetamine yields approximately 12 mg of active dextroamphetamine 2
Adderall contains a 3:1 ratio of dextroamphetamine to levoamphetamine salts, so the dextroamphetamine component is approximately 75% of the total dose 4
Recommended Starting Approach
Start with 10 mg of Adderall (either immediate-release twice daily as 5 mg doses, or extended-release once daily), then titrate upward in 5-10 mg increments weekly based on symptom control. 4, 5
For adults, the American Academy of Child and Adolescent Psychiatry recommends starting at 5 mg of immediate-release amphetamine formulations with titration upward in 5-10 mg intervals each week until symptoms are controlled 4
The maximum daily dose for adults is generally 40 mg for mixed amphetamine salts, though some patients may require up to 50 mg with clear documentation of need 4, 5
Why 20 mg May Be Too High Initially
Starting at 20 mg represents a significant jump from the equivalent of 12 mg dextroamphetamine base from the 40 mg Vyvanse dose 2
The pharmacokinetic profile shows that equimolar doses of lisdexamfetamine and D-amphetamine produce similar peak concentrations and total exposure, but the immediate-release nature of Adderall IR may produce more pronounced peak effects 2
A more conservative starting dose minimizes the risk of cardiovascular side effects (increased blood pressure and heart rate) and other adverse effects like agitation or insomnia 4, 2
Titration Strategy
Begin with 10 mg daily (either as 5 mg twice daily for IR or 10 mg once daily for XR formulation) 4, 5
Increase by 5-10 mg weekly based on clinical response, monitoring with standardized ADHD rating scales when possible 4
Monitor blood pressure and pulse quarterly in adults on stimulants 5
If 10-15 mg proves insufficient after 1-2 weeks, then consider increasing to 20 mg, but this should be based on documented inadequate response rather than assumed equivalence 4
Important Caveats
The patient's ability to communicate symptoms does not justify skipping conservative dose titration, as cardiovascular and psychiatric side effects can occur even in articulate patients 4, 2
Document clearly if higher doses (approaching 40-50 mg) are needed, noting that symptoms could not be controlled at lower doses and that higher doses are not producing side effects 4
Consider that approximately 70% of patients respond to a single stimulant, and if inadequate response occurs at maximum doses, switching to an alternative stimulant may be more appropriate than exceeding recommended maximums 4, 5