In order for physicians and other healthcare practitioners to recommend a treatment or healing practice to patients, they need evidence that it is safe and effective. With respect to safety, there have been no reported negative effects from Reiki  in any of the research studies. This is understandable given that no substance is ingested or applied to the skin, and Reiki touch is non-manipulative (and can be offered off the body when needed).
That leaves the question: is Reiki effective? Or more precisely, from a research perspective, what is Reiki effective for?
A Reiki practitioner would answer that question by saying, "Reiki is effective for restoring balance, which can show up in a number of ways, depending on the current need of the individual." That's not an answer that appeals to medical researchers, who are used to studying treatments for specific illnesses rather than treatments to promote wellness or restore balance.
Respected medical research is designed to address very specific questions. Although conventional medicine has long included a concept of homeostasis, or systemic balance, there has historically been no clear definition of this concept that can be used to test the hypothesis that Reiki promotes balance. Given the vagueness of the term stress and the differences in human bodies and the circumstances in which they live and function, how would science measure an individual's balance?
In view of the research dilemma posed by the ambiguity of the term stress, brain researcher Bruce McEwen of Rockefeller University has proposed a new model using the terms allostasis/allostatic load. Allostasis refers to the body's attempt to protect itself and regain homeostasis, and allostatic load refers to the damage that accumulates when those attempts are poorly managed and the stress response runs amuck.
Besides helping people recognize the difference between useful and harmful stress and how to reduce the latter, McEwen's model puts forth a series of measurements that researchers can use to determine both the effect of stress on the system and the response to stress-reducing modalities. Reiki has not yet been studied in this way.
To date, the primary outcomes studied in Reiki resarch have used measures for pain, anxiety, and stress, including heart rate, blood pressure, salivary cortisol, as well as measures for job burnout and caring efficacy. More specific measures have been used to evaluate outcomes for stroke rehabilitation, depression, and other chronic health conditions. Given the relatively subtle and complex nature of Reiki practice, these measures may not adequately capture the lived experience of those receiving Reiki. Measures that incorporate quality of life, patient satisfaction, and stress reduction may have the greatest potential for demonstrating the benefits of Reiki practice.
Studying modalities such as Reiki brings up other questions. The randomized controlled trial is well suited to studying the impact of pharmaceutical products (although recent developments have shown that even this line of inquiry can be manipulated).
But is the linear simplicity of the randomized controlled trial well suited to studying therapies that clearly elicit complex, multileveled, rapid and lasting responses such as is seen with Reiki? Many respected researchers think not, and a dialogue about how best to study Reiki and other integrative therapies  and healing practices has begun. Systems theory is increasingly seen as providing a more viable approach to study the web of interactions involved in integrative therapies. Qualitative research may also provide a broader lens in generating relevant data.
A unique confounding variable in Reiki research is controlling for the effects of human touch. Do Reiki recipients have improved outcomes because they have received sustained human touch? Furthermore, how do you create a placebo standard for a hands-on healing technique? In 1999, placebo standardization was introduced into Reiki research, demonstrating that study participants couldn't differentiate between the identity of placebo and Reiki practitioners. The addition of a placebo arm in Reiki research strengthens study design and addresses the confounding variable of human touch.
Another obstacle to Reiki research is the inability of contemporary technology to document the existence of the biofield, much less study its makeup or measure changes in it. Superconducting quantum interference devices (SQUIDs) measure extremely small magnetic fields and may in the future prove useful to this study. The speed with which technological advances are being made may mean that the needed technology is on the brink of development. However, it is also possible that Reiki or biofields lie outside the bioelectromagnetic spectrum.
Fortunately, it is not necessary for science to document the existence of either Reiki or the biofield in order to measure the impact of Reiki on the human system (aspirin was used for 70 years before science began to understand how it works). Although some effects of Reiki are measurable, such as improved heart rate and blood pressure, many commonly reported benefits of repeated Reiki sessions, such as a sense of spiritual connection and enhanced self-esteem, may not be quantifiable. It is still important to document these reported benefits.
Patients who feel more spiritually connected and who simply feel better about themselves may well be patients who are easier to treat and who are better equipped to follow treatment protocols. In this way, Reiki might be shown to significantly, albeit indirectly, impact medical outcomes by supporting the ability of patients to access conventional medicine and gain a heightened awareness of their own needs.
While the debate on how best to study integrative therapies such as Reiki is gaining steam, research attempts have been and continue to be made. Nonetheless, research into Reiki is just beginning. The National Center for Complementary and Integrative Health  (NCCIH) of the National Institutes of Health  (NIH) has completed five studies looking at Reiki's ability to benefit people with diabetes, advanced AIDS, prostate cancer, fibromyalgia, and stress.
Other published studies have looked at the effect of Reiki on measures of stress hormones, blood pressure, heart rate, and immune responsivity, and on subjective reports of anxiety , pain and depression . The studies to date are typically small, and not every study is well designed. However, overlapping data from some of the stronger studies support the ability of Reiki to reduce anxiety and pain, and suggest its usefulness to induce relaxation, improve fatigue and depressive symptoms, and strengthen overall wellbeing . The Cochrane Database of Systematic Reviews contains a review on the use of touch therapies (including Reiki) for pain and a protocol for use of Reiki for psychological symptoms.
Reiki has been increasingly offered as part of workplace wellness programs to address burnout and improve skills in healthcare and other industries, as well as in university wellness centers.