Reiki is, simply put, good energy!! The practise of Reiki is the taking away of bad energy, eg pain, and replacing it with good energy, ie healing. It comes from the power of good in the universe and can be used by anyone who has been initiated into its light. Reiki will only ever do good and can help to heal everything from headaches to world catastrophes!!
Anyone can become a channel for the 'Universal Golden Ray of Healing Light' that is Reiki. Once having been initiated by a qualified Reiki Master/ Teacher, you have access to it for life. It is then up to you - and the universe - how you use it.