Belief in the healing power of prayer is found in various religious traditions. Spiritually grounded clinical interventions, such as intercessory prayer (IP), need to be understood in a broader sense. This essay features the IP trials, observing the controversial relationship between inconsistent results and allegedly inadequate methods and theoretical hypothesis. A survey of the literature was conducted including publications indexed until September 2013, focusing on the trials developed in the field and on the critics about the methodological design. Recent meta-analyses and multicenter studies found inconclusive results in the investigation of IP. Clinical trials on IP present some methodological difficulties: The intervention is not fully controlled; the primary outcome is not properly defined; and the theoretical models seem inconsistent. The "non-local consciousness" model may be appropriate for studies of IP. Directions for future research: greater emphasis on the evaluation of the effectiveness of this intervention in animal models; selection of subjects and healers who have previous connection; considering the hypothesis of non-local consciousness in the study design.
