Has prayer been tested in a clinical trial?

Yes, more than once. Researchers have run randomized trials in which patients were assigned by chance to be prayed for by strangers. The patients did not know which group they were in.

The largest was published in the American Heart Journal in 2006. It enrolled 1,802 patients having coronary artery bypass surgery at six US hospitals. Prayer was offered for fourteen days, starting the night before surgery. Some patients were told they might or might not receive prayer. In that set, complications within 30 days occurred in 52 percent of those prayed for and 51 percent of those not prayed for. The authors concluded that intercessory prayer itself had no effect on complication-free recovery.

A second large trial was published in The Lancet in 2005. It randomized 748 patients having cardiac procedures. It reported that masked prayer did not significantly improve clinical outcome.

A Cochrane systematic review pooled ten trials with 7,646 patients. It called the findings equivocal. The evidence, it said, does not support a recommendation either for or against the use of intercessory prayer.

None of these trials studied cancer. None of them can say what prayer means to a person. They tested one narrow question about medical outcomes you can measure. On that question, the answer has been consistent.