placebo effect

I don't have a clue what the placebo effect is, or how it works. I read articles below, after Geni mentioned it in another thread. I don't understand how the placebo effect works. Why it is allowed to be administered by the medical professionals. Why do scientists seem to accept the effect...

Are there placebo effects in studies on animals? If so, are there any references to studies which show this?

Brain Scans Show Placebo Effect Is All in the Head My first reaction was that this is a tautology but the article is kind of interesting. I wonder if one can become a 'placebo addict'.

FDA Approves Sale of Prescription Placebo (Okay, okay, it's an Onion article, but it just seemed sooooooo appropriate.)