The strongest research conducted as of 2008 has failed to demonstrate that Reiki is an effective treatment for any condition. This systematic review assessed this evidence base, finding nine studies which fit their selection criteria. A modified Jadad score of methodological quality was used, taking into account the difficulty of blinding practitioners. Non-randomized studies were excluded, as the potential for intentional or unintentional bias in such studies is large, rendering the results uninterpretable. Overall, the methodological quality of the evidence base was found wanting, with even high-ranking studies failing fully to control for placebo effects and most studies suffering "methodological flaws such as small sample size, inadequate study design and poor reporting." As trials with such flaws are known to be likely to show exaggerated treatment effects, there is insufficient evidence to indicate that reiki is effective as sole or adjuvant therapy for any medical condition, or that it has any benefits beyond possible placebo effects.
Does it work? Yes. Reiki works as well as any other placebo medicine. It works primarily by the power of suggestion and classical conditioning, both of which can bring about physiological changes in the believer or the open-minded skeptic who knows little about placebo energy medicine.