To date, there has never been a shred of strong, verifiable evidence of the efficacy of prayer. There is much debate as to how to test prayer, or even whether it can be tested, but two lines of evidence seem to speak to the issue.
First, many medical studies have been performed to try to understand the effects of praying on an individual who is seriously sick or ill. Based on all meta-analysis done on these studies since the early 20th century, the best effect that prayer can be said to have is to put pressure on study subjects: patients who don’t know whether they’re being prayed for do much better than patients who are told they are being prayed for. Presumably, being told that people in a study are praying for you and if you don’t get well very soon you’ll be counted as evidence against God’s ability to intervene in the world is not the best way to put a patient at ease and let them heal.
The important point to derive from prayer efficacy studies, however, is that ill people who have no idea whether they’re being prayed for aren’t affected in the slightest by the presence or absence of prayer. This would seem to indicate that some people get better, and sometimes they have been prayed for. It’s part of humanity’s deep-seeded desire to see intelligence manipulating the world that makes many of us retrospectively attribute healing to the so-called power of prayer.
Another good piece of evidence against the power of prayer is the track records of faith healers. These charlatans (whether intentionally so or not), make people believe that they can, through faith and prayer, heal the sick or dying. Well, alright, if someone wants to claim that and do a lot of showmanship and crowd-culling to back it up, that’s fine. But such a person must come up with an explanation for one outstanding fact. Why does God hate cripples and wounded soldiers so much?
By asking this question, I mean to make the point that the only maladies ever healed by faith healers or prayers are those that we know can go into spontaneous remission anyway. If you pray for a cancer patient, they might rally. If you pray for a child with a severe fever, they might do the same. However, cancer patients and children with fevers pull through all the time and without explanation. If you want to attribute this to the will of God, then you must explain why no one with a severed spinal cord or a severed limb ever spontaneously gets better.
In short, the advocates of prayer claim, in spite of all available evidence, that they’re mutterings can help people by evoking an all-powerful deity. To support this, they point to the fact that sometimes people beat diseases that people are known to beat anyway. This seems pretty shabby as a basis of evidence.
As an atheist, I’ll merely leave my finger pointed firmly in the direction of those with maladies that medicine and spontaneous remission cannot cure. If people start growing back arms or having their spinal cords miraculously spliced back together while someone is praying for them, then that will be the time to take notice and allow the efficacy of prayer a potential claim to the credit.