What is the equivalent dose of Vyvanse (lisdexamfetamine) for a patient taking 20mg of Adderall (amphetamine and dextroamphetamine)?

Equivalent Dose of Vyvanse for 20mg Adderall

The equivalent dose of Vyvanse (lisdexamfetamine) for a patient taking 20mg of Adderall (amphetamine and dextroamphetamine) is approximately 50mg.

Conversion Rationale

Lisdexamfetamine is a prodrug that is converted to dextroamphetamine in the body through rate-limited enzymatic hydrolysis. This conversion process affects how we calculate equivalent dosing:

• Lisdexamfetamine is therapeutically inactive until metabolized in the body to dextroamphetamine, which is responsible for the drug's activity 1
• The usual starting dose for Vyvanse is 20-30mg once daily, which can be increased by 10mg weekly to a maximum of 70mg daily 2
• For Adderall (mixed amphetamine salts), the usual starting dose is 10mg once daily, which can be titrated by 5mg weekly up to 50mg 2

Pharmacokinetic Considerations

When converting between these medications, it's important to understand their pharmacokinetic differences:

• Lisdexamfetamine follows linear pharmacokinetics at therapeutic doses (30-70mg) 1
• The lisdexamfetamine molecule is enzymatically hydrolyzed, primarily in the blood, to active dextroamphetamine 3
• Studies show that lisdexamfetamine has approximately a 0.6 ± 0.6 hour longer lag time and reaches peak levels about 1.1 ± 1.5 hours later than direct dextroamphetamine administration 4
• Despite this delayed onset, the maximal concentrations and total exposure (AUC) are similar between equivalent doses 4

Dosing Algorithm

For converting from Adderall to Vyvanse:

1. Start with the current Adderall dose (20mg)
2. Apply a conversion factor of approximately 2.5:1 (Vyvanse:Adderall)
3. Therefore: 20mg Adderall × 2.5 = 50mg Vyvanse

Clinical Considerations

When switching between these medications, be aware of:

• Lisdexamfetamine provides a duration of action of approximately 12-14 hours (13 hours in children, 14 hours in adults) 3
• The side effect profiles are similar between the medications, with common adverse effects including decreased appetite, insomnia, headache, irritability, and weight loss 1, 5
• Lisdexamfetamine was developed to provide an extended duration of effect that is consistent throughout the day 3

Monitoring and Adjustments

After initiating Vyvanse:

• Monitor for effectiveness in controlling ADHD symptoms throughout the day
• Assess for common side effects including appetite suppression, insomnia, and headache 6
• If needed, dosage can be adjusted in 10mg increments at weekly intervals 2
• Maximum recommended dose is 70mg daily 2

Common Pitfalls to Avoid

• Avoid assuming that the conversion is exactly 1:1 by milligram weight, as the medications have different pharmacokinetic properties 4
• Don't overlook the delayed onset of action with lisdexamfetamine compared to immediate-release Adderall 4
• Remember that individual patient response may vary, and some patients may require dosage adjustments based on clinical response and tolerability