Reiki is effective on any existence in the universe. However, it is important to keep your consciousness clear and not to tune to low or rough wave. Reiki revitalizes Life Energy and enhances the natural healing ability. Medicines and other techniques can be as effective as it should be by the synergistic effect with Reiki. Transcends space and time - Reiki enables you to perform healing in the distance, towards the past or the future, by using the symbols.
All existence is a wave.
Others and yourself are identical in reality and there is no wall of space and time. Although the symbols are useful, they become unnecessary once you understand the essence of the energy. Purify karma -Reiki is useful for purifying karma or healing trauma. Bad karma from the past should be atoned for true happiness. To do so, your karma from the past has to be extinguished by repeating self-cleansing with Reiki energy.