Vyvanse can cause weight loss and potentially affect menstruation, especially in cases where a significant amount of weight has been lost rapidly. This weight loss may be due to a decrease in appetite caused by the medication. It is important to consult with a healthcare professional if you are experiencing unintended weight loss, as it may lead to nutritional deficiencies. Additionally, significant weight loss can disrupt menstrual cycles, potentially causing them to stop altogether. It is advisable to speak with a doctor about any concerns regarding weight loss and to discuss possible adjustments to the medication.