The Big Picture

Consuming orange juice significantly impacts the effectiveness of amphetamine-based medications such as Adderall or Vyvanse, whereas there's a negligible effect on Ritalin or Concerta users, as these medications are more stable in acidic environments.

In Depth

There are actually two major pathways by which orange juice impacts ADHD medication.

Firstly, Adderall and Vyvanse are 'ionized' by a small change in acidity caused by Vitamin C. Once they are ionized, its much harder for them to cross the intestinal wall meaning some portion of the dose never reaches the bloodstream.

Secondly, and more importantly, Vitamin C is particularly good at turning urine acidic. Once urine is acidic, the kidneys filter out Adderall and Vyvanse and they cannot be reabsorbed. Ending your dose prematurely.