If it works better than blind chance, as many claim it does, then we should be able to measure and demonstrate that. If we measure it and find that it works at about the rate of chance, shouldn't we conclude that it doesn't work? If you're honest, you'll realise very quickly that this is exactly what we find!
When new medicines are developed, several trials are conducted to determine whether the medicines perform better than a placebo. Rigorous controls are put in place to rule out bias, and we all reap the benefits of this. We know this method of verification works. What many believers don't realise is that the same kinds of trials have been conducted with prayer, and the most conclusive results have all suggested that prayer does not work better than chance. One study in particular suggested that when people know they are being prayed for, they may actually get worse!
What if one particular religion was true, and thus only that religion's prayers were answered? Wouldn't that become quite obvious over time? Have a look around and tell me which religion has the highest rate of answered prayer... I'll wait.
Believers cite anecdotes of miracles and small successes, failing to appreciate that the same results are achieved by those who don't pray (or those who pray to other gods) and at exactly the same rate. If God cannot heal people at better than the rate of chance, why pray? How does prayer differ from merely talking to yourself? What if that's actually all you're doing?
Meanwhile, medicines can heal people at far better than the rate of chance, and since the invention of modern medicine people have been cured of many ailments that in times past resulted in the deaths of millions (despite any uttered prayers). Should we conclude that God simply decided not to heal those people in earlier times? Was it God's plan that people should die from various diseases for thousands of years, but then he suddenly made an exception once modern medicine came along? Is modern medicine a perversion of God's natural order? Some religious people think so - and will even go as far as refusing some medical treatments on the basis of their belief that those treatments are forbidden by God!
But if these modern cures were always there just waiting to be discovered, why didn't God share them with us earlier? Better yet, why didn't he make us immune to the diseases from the beginning, given that such immunity was clearly possible via medicine? In other words, if we can do it, why couldn't God do it? If God could have done it, why didn't he? I don't have an answer to these questions (other than to suggest God probably doesn't exist), but I can tell you what would happen if a human parent had access to such cures but withheld them from their children. Most of us have no problems identifying immoral behaviour such as negligence when it involves humans, but believers want to make exceptions for their gods. They argue that gods shouldn't be held to a human standard, but shouldn't they be at least held to a higher one?!
Why do people only pray for things the doctor can't cure? Would you pray for God to cure a headache, or do you think paracetamol would work faster? Perhaps God is too busy for such minor ailments as headaches. What about heart disease? Prayer or medication - which would you choose? Of course believers want to say "both". But remember that just 200 years ago people didn't have that choice. They only had prayer. Did they fare better or worse? Why?