Equivalent Dose Conversion: Adderall XR 20mg to Vyvanse
For a patient taking Adderall XR 20mg, the equivalent dose of Vyvanse is 50mg once daily, based on an approximate 2.5:1 conversion ratio. 1
Conversion Algorithm
The conversion from Adderall XR to Vyvanse follows a straightforward mathematical approach:
- Apply a 2.5:1 ratio (Vyvanse:Adderall) when switching between these medications 1
- For Adderall XR 20mg: multiply by 2.5 to get Vyvanse 50mg 1
- This conversion accounts for the prodrug nature of lisdexamfetamine, which must be metabolized to dextroamphetamine in the bloodstream 2, 3
Pharmacokinetic Rationale
The conversion ratio is supported by the distinct pharmacokinetic profiles of these medications:
- Lisdexamfetamine (Vyvanse) is a prodrug that is therapeutically inactive until red blood cells convert it to dextroamphetamine through hydrolytic activity 2, 3
- Time to peak effect differs: Vyvanse reaches peak dextroamphetamine levels at approximately 3.5-4.4 hours post-dose, compared to the bimodal release of Adderall XR 2
- Equivalent amphetamine exposure is achieved at higher milligram doses of Vyvanse due to the molecular weight difference (lisdexamfetamine dimesylate contains approximately 29.5% dextroamphetamine by weight) 2
Dosing Parameters and Adjustments
After conversion to Vyvanse 50mg:
- Dosage range: The recommended starting dose for Vyvanse is 30mg, with titration in 10-20mg increments at weekly intervals 1, 2
- Maximum dose: 70mg once daily 1, 2
- Administration: Take once daily in the morning with or without food; avoid afternoon doses due to insomnia risk 2
- Titration flexibility: If 50mg is not well-tolerated or inadequately effective, adjust in 10mg increments weekly 1
Important Clinical Caveats
Renal impairment requires dose adjustment: In severe renal impairment (GFR 15-30 mL/min/1.73 m²), maximum dose is 50mg daily; in end-stage renal disease (GFR <15 mL/min/1.73 m²), maximum is 30mg daily 2
Duration of effect: Vyvanse provides full-day coverage (up to 12-13 hours) due to its prodrug formulation, which may differ from the patient's experience with Adderall XR 3
Onset differences: Expect a slightly delayed onset with Vyvanse (approximately 1 hour longer lag time to therapeutic effect compared to immediate amphetamine formulations) 4