Mydayis 37.5 mg is approximately equivalent to Vyvanse 50 mg
Based on FDA pharmacokinetic data, Mydayis 37.5 mg provides comparable amphetamine exposure to mixed amphetamine salts extended-release (MAS-ER) 25 mg plus an additional immediate-release 12.5 mg dose given 8 hours later, totaling 37.5 mg of amphetamine salts daily. Since Vyvanse (lisdexamfetamine) is a prodrug that converts to dextroamphetamine, and considering the typical conversion ratio, Vyvanse 50 mg would be the closest equivalent dose 1.
Rationale for Dose Equivalence
Pharmacokinetic Comparison
The FDA label for Mydayis explicitly demonstrates that a single 37.5 mg dose produces plasma concentration profiles of both d- and l-amphetamine comparable to MAS-ER 25 mg followed by immediate-release amphetamine 12.5 mg administered 8 hours later 1. This triple-bead formulation provides:
- Immediate-release component
- Two delayed-release components (releasing at pH 5.5 and pH 7.0)
- Total amphetamine base equivalence of 23.5 mg for the 37.5 mg dose 1
Vyvanse Conversion Considerations
Lisdexamfetamine is converted to dextroamphetamine after enzymatic hydrolysis in red blood cells 2. The typical dosing range for Vyvanse is 20-70 mg daily, with starting doses of 20-30 mg 2.
The conversion approximation:
- Mydayis 37.5 mg contains 23.5 mg amphetamine base equivalent
- Vyvanse 50 mg provides roughly comparable amphetamine exposure when accounting for the prodrug conversion
- This represents a reasonable 1:1.3 ratio (Mydayis to Vyvanse)
Clinical Titration Approach
When switching between these medications:
- Start with Vyvanse 50 mg once daily as the initial equivalent dose
- Monitor response at 1-2 weeks using ADHD rating scales and functional assessments
- Adjust in 10 mg increments of Vyvanse (up to maximum 70 mg) based on symptom control and tolerability 2
- Assess duration of coverage: Mydayis provides up to 16-hour coverage 3, while Vyvanse typically provides 13-14 hour coverage 4
Important Caveats
Duration differences matter clinically: Mydayis 37.5 mg is specifically designed for extended 16-hour symptom control 3, whereas Vyvanse's duration is typically 13-14 hours 4. If the patient requires late-day coverage that Mydayis was providing, you may need:
- Vyvanse 60 mg for enhanced duration, or
- Addition of a small immediate-release amphetamine supplement in late afternoon
Individual variability exists: The 3:1 ratio of dextro- to levo-amphetamine in Mydayis differs from Vyvanse's pure dextroamphetamine formulation. Some patients may respond differently to these enantiomer ratios, requiring dose adjustment beyond the calculated equivalent 1, 5.
Monitoring requirements remain standard: Check blood pressure, pulse, weight, and ADHD symptoms weekly during titration, then monthly during maintenance 6. Both medications carry similar cardiovascular and psychiatric monitoring requirements 1.