I have adhd and take 20mg of vyvanse (lisdexamfetamine) and sometimes the vyvanse (lisdexamfetamine) gives me really bad anxiety. why is this?

Known effect: Stimulant medications have a long list of known side effects, of which irritability is one. Careful monitoring by your physician is warranted when taking these medications, and it isn't uncommon for a person to try a few different stimulants to find the one which produces the desired effect (focus, concentration) without troublesome side effects.

STIMULANT: Vyvanse (lisdexamfetamine) is a stimulant so this may be the issue and may need to adjust the dose or make sure that you eat before taking the medication.