Actually, the most recent double-blind study showed that prayer DID have an effect...at least on those patients who knew loved ones were praying for them.
Unfortunately, it actually made those patients worse.
Induced performance anxiety, perhaps?
I'm familiar with this study but I'm not sure the difference was statistically significant. That being said, there are no controlled studies I'm aware of that show any medical efficacy for prayer. It might make the praying friend or relative feel better but it doesn't seem to benefit the patient.