What is the ruling on using henna for the entire body and for the head?

There is no harm in applying henna to any part of the body for medicinal purposes. It has been authenticated that whenever the Prophet, peace and blessings be upon him, was afflicted by a sting or a thorn, he would apply henna to it. He also used to recommend applying henna to anyone who complained of pain in their leg. The Companions also used to dye themselves with henna.