My son is almost 4 and weights 30 pounds and is on vyvanse, (lisdexamfetamine) how long before i should notice a change?

ADHD: Usual age of starting medication for hyperactivity is 5 year, when child starts school. In rare occasion, given at an earlier age, e.g. Child is uncontrolably hyperactive, disruptive, goes to daycare, etc. Decision is made considering all/many things and after thorough evaluation. Vyvanse and other stimulant medications start working on the same day in most of the patients.

Usually fast acting: The stimulant class of medication acts quickly in the system. It is not as reliant on height or weight as the individual's ability to quickly metabolize the medicine. I usually tell families to look for changes after the first 72 hours of medication use. Even subtle changes may be relevant, both positive and negative changes (side-effects and poor reactions to medication.