Hi! Welcome to HCM! Since you have not mentioned about the nature of blood test your doctor wants to do, it is very difficult to give you explanation about its interference. However, Vyvanse is commonly prescribed by the doctors for a psychological condition known as Attention deficit hyperactivity syndrome. Besides, it is most oftenly misused by the atheletes for enhancing their performance this is known as doping. It increases performance in strength exercise, alertness, aggressiveness & decreases fatigue. It is a habit forming drug should not be used for other than a genuine clinical indication. It is known to cause the following side effects: Insomnia, impaired decisions making, anxiety & tremors , headache, confusion , restlessness, hypertension. I hope I tried to answer your queries. REgards!