It’s not as strong as you might think:

The largest study of the medical power of prayer found secret prayers on patients’ behalf didn’t reduce complications in heart surgery and there was a 14 percent higher chance of problems when patients were aware of the prayers.

The research, appearing in the April issue of the American Heart Journal, followed 1,802 patients undergoing coronary bypass surgery at six hospitals. Of those, one-third weren’t prayed for, another third were prayed for without their knowledge by three U.S. Christian congregations, and the rest knew they were the subject of prayers by the church members.

The researchers, led by Herbert Benson and Jeffery A. Dusek of Harvard Medical School and Mind/Body Institute, found that people who knew they received intercessory prayers had the highest chance of complications of the three groups, 59 percent. Among patients who didn’t know whether they were the subject of secret prayers, complications occurred in 52 percent of those who were prayed for and 51 percent of those who weren’t.

The death rates for 30 days after surgery were similar across all the groups, the two-year study showed.

So if I get sick, it doesn’t matter whether you pray for me or not.  But for God’s sakes, don’t tell me that you’re praying for me, because you’re only hurting my chances.