Hi, Vyvanse contains lisdexamfetamine dimesylate which is a long lasting nervous system stimulant that belongs to the group of drugs known as methamphetamines. This drug is a controlled substance that means it has the potential for abuse or dependence. Vyvanse works by altering the balance chemicals in the brain and increasing norepinephrine and dopamine levels. Norepinephrine is a stimulant. Dopamine is a naturally occurring substance that affects the pleasure and reward aspect of the body. Someone may feel the medication working within a few days, but it usually takes a few weeks to achieve the full effect. The doctor may need to adjust the dose to get the desired results. The amphetamine stimulant lisdexamfetamine like Vyvanse has been reported to cause muscle pain, cramping, stiffness and back pain without other serotonin related symptoms. With the higher dose of Vyvanse may aggravate some side effects that usually can disappear when someone s body will adjust the new dose. But, if these side effects persist for long or bother your son, then he should consult his doctor promptly for medical help. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Zubayer Alam, General And Family Physician