What Causes Hair Loss While On Medication For ADHD?

I was prescribed adhd medicine about 1 1/2 yrs ago. ( 1yr on concerta and now vyvanse for 6 months). I am now having diffuse hair loss. I have had blood work done and everything seems normal. my psych. and the pharmacist say my adhd meds would not be causing this. But do you think it would be a possibilility even though its not in the side effects book yet?