Does Vyvanse Cause Sunburn Looking Rash On Forearms And Calves?

My teenage son takes Vyvanse for ADHD. After a recent trip to the pool, he developed a sunburn looking rash on his forearms and near his calves. He wears a Tshirt when he swims, so his torso and back were not affected. Is this a reaction to his medication such as sun sensitivity? He got burned very badly approx 4 years ago in which his forearms had water blisters. This seems to be the main area where the rash occurs. It has also happened with other ADHD medicine such as Concerta and Adderall.