Fioricet is a powerful prescription drug, which is used to bring relief in tension headaches, mainly because it relaxes muscle stiffness that causes pain in head, ranging from moderate to severe. It must be remembered that a tension headache is somewhat different from migraine. Fioricet is a powerful prescription drug, which is used to bring relief in tension headaches, mainly because it relaxes muscle stiffness that causes pain in head, ranging from moderate to severe. It must be remembered that a tension headache is somewhat different from migraine. Fioricet is a powerful prescription drug, which is used to bring relief in tension headaches, mainly because it relaxes muscle stiffness that causes pain in head, ranging from moderate to severe. It must be remembered that a tension headache is somewhat different from migraine.