The existence of Reiki energy has not been scientifically proven, and anecdotal evidence of the effectiveness of Reiki therapy is commonly ascribed to the placebo effect and a combination of post hoc reasoning and the regressive fallacy by critics. Proponents of Reiki claim that they can detect and manipulate this energy, but a means to measure it or even objectively demonstrate its existence to the satisfaction of the scientific community has yet to be found. The predominant opinion among the scientific community is that the sensations felt by practitioners and patients of Reiki are psychologically subjective or the result of self-deceit.

Many practitioners of traditional Chinese medicine are also sceptical of the claims of Reiki practitioners.

Doctors, academics, and consumer advocates have expressed concern when patients with serious diseases such as cancer choose Reiki solely as a means of treatment over trained doctors. In some cases people reject conventional medicine completely and solely practice Reiki, and this is deemed as a highly untrustworthy and potentially dangerous practice even within the Reiki and wider alternative health community. While it is understandable for patients to seek non-mainstream remedies when conventional options seem ineffective or untrustworthy, many doctors say that Reiki, like many other forms of alternative medicine, is simply exploiting the fear and hope of people with serious illnesses for money while offering no demonstrable help. The response from Reiki practitioners is that Reiki is a reliable and effective treatment that is being unfairly dismissed by conservative Western scientific research. Some Reiki teachers and practitioners advocate a complementary approach to conventional medicine - holding it as the most prudent and responsible means of treating any illness - much like many other alternative medicine advocates suggest.