Half-Life of Adderall XR
The half-life of Adderall XR (extended-release mixed amphetamine salts) is approximately 12 hours, with effects lasting up to 12 hours after a single dose. 1
Pharmacokinetic Profile
Adderall XR has a unique two-stage delivery system that provides both immediate and delayed release of amphetamine salts:
- Initial absorption phase: First half of the dose is released immediately upon ingestion
- Delayed release phase: Second half of the dose is released approximately 4 hours later
- Peak plasma concentration: Occurs approximately 8 hours after dosing 1
- Average maximum concentration (Cmax): 23.5 ng/mL for the extended-release formulation 1
This biphasic release pattern allows for once-daily dosing while providing coverage throughout the day for ADHD symptoms.
Comparison with Immediate-Release Formulation
The pharmacokinetic profile of Adderall XR differs significantly from immediate-release Adderall:
- Immediate-release Adderall: Reaches maximum plasma concentrations (Cmax of 36.6 ng/mL) at approximately 3 hours after administration 1
- Adderall XR: Provides comparable bioavailability to immediate-release tablets but with a more gradual absorption profile 1
- The bioavailability and pharmacokinetic profile of Adderall XR 20 mg is comparable to Adderall IR 10 mg taken twice daily 2
Factors Affecting Absorption and Half-Life
Several factors can influence the absorption and effective half-life of Adderall XR:
- Food effects: High-fat meals can significantly affect early drug exposure of Adderall XR, resulting in lower amphetamine concentrations during the first 8 hours after dosing 3
- Individual metabolism: Elimination half-life may vary between individuals based on factors such as age, liver function, and urinary pH
- Weight and body composition: May affect distribution and elimination
Clinical Implications
Understanding the half-life of Adderall XR has important clinical implications:
- The 12-hour duration of action allows for once-daily dosing, typically in the morning
- The extended duration minimizes the "wearing off" effect often seen with immediate-release stimulants
- Evening dosing should be avoided as it may interfere with sleep due to the long half-life
- Consistent timing of administration relative to meals may help maintain predictable drug levels
Caution
When prescribing Adderall XR, be aware that:
- The long half-life may lead to accumulation with repeated dosing, particularly in patients with impaired metabolism
- The extended duration means side effects may persist longer compared to immediate-release formulations
- Morning administration is recommended to minimize sleep disturbances
The 12-hour half-life of Adderall XR provides the clinical advantage of once-daily dosing while maintaining therapeutic effects throughout the day for patients with ADHD.