Scientifically measuring a supernatural process is essentially impossible. What counts as a prayer? Should the praying people meet the patients, or is it sufficient that they know the patients' names? Or maybe they should see photographs of the patients. Do they need to pray for a specific outcome? The cardiac study asked the praying people to include a specific phrase: "for a successful surgery with a quick, healthy recovery and no complications" [source: Carey]. Is it possible (and ethical) to prevent the friends and family of the patients from praying for them, which could affect the results of the study? Does it matter what religion or denomination the prayers are given in?
In 2001, the British Medical Journal published a satirical study on "retroactive intercessory prayer" that showed positive results on the outcomes of patients in the past. In other words, prayer was powerful enough to effectively time travel backward and affect patient health. The study was intended as a critique to point out the logical impossibility of scientifically testing the effect of prayer, but proponents of faith healing continue to cite the study's results as evidence of its efficacy.
There are plenty of anecdotes about people experiencing seemingly miraculous cures after being prayed for. Anecdotes, of course, don't offer conclusive evidence and can't be reliably studied. People do get better, sometimes unexpectedly and from serious illnesses, and someone who has religious beliefs is likely to make a connection.
It is possible that some of the benefit from prayer is due to the placebo effect — when fake treatments that appear real have actual positive effects for a patient — which often has overestimated effects, but is real [source: Justman]. It's also possible that, contrary to the coronary study's results, people praying for someone can bring them comfort, reducing anxiety, stress and the medical complications prolonged stress can cause. And just like with individual faith healers, patients can temporarily feel better, convinced that the power of prayer has healed them. Anecdotes about prayer healing rarely mention follow-ups to see if the patient had truly and permanently recovered.
But in the end, the statistical evidence is inconclusive. Prayers will not hurt you (unless you're a cardiac patient and you know people are praying for you, apparently), but for medical problems your first stop should be a medical doctor.