I lost 12 lbs in 6 weeks, not too long after starting vyvanse (lisdexamfetamine). should i be concerned? when would i need to worry?

Depends: on what your wt was prior to the wt loss. Obviously you are not anorexic or bulimic if you are concerned about it and I am assuming your eating habits have not changed. I would be concerned if the wt loss continues at this point and would probably let your prescribing doc know as wt loss is a common side effect and it is important to monitor your wt from a well calibrated scale. Take care