While diarrhea is not a common side effect of Vyvanse, it is possible for it to occur. If your child is experiencing diarrhea while you are breastfeeding and taking Vyvanse, it is important to monitor their overall condition. Typically, diarrhea caused by medication is mild and temporary. If you have any concerns or notice any worsening symptoms, it is important to consult with a healthcare professional for further guidance.