The Placebo Effect on Pain
Following is a description of how one goes about demonstrating this effect as well as an investigation of the chemical mechanism

The lovely young subjects are told that they will be participating in a trial to test the effects of a pain killer on their pain threshold. They then allow the professional researcher to slip a fairly thick needle into the vien in the crook of the elbow.

"I'm going to stomp on your foot now, Cassandra, and I want you to rate the pain on a scale from one to five," whispers the good scientist.
The scientist stomps and she screams "four."
"Igor, flip the switch to that pump over there. You are now receiveing the pain killer, Cassandra. In a moment I will again stomp on your foot, and perhaps with the fury of MethRat." (saline, merely, is pumped into Cassandra)
The scientist exerts a fair amount of force, and suddenly too, on the poor girl's foot, to which she replies "Two."
"You will now receive another dose of pain killer, young lady," hisses the scientist with an arbitrary amount of triumph. (Igor switches on the pump again, however this time Cassandra receives a dose of Naloxone.

Naloxone is an opiod antagonist. Opiods include heroin, hydrocodone(codeine), and methadone, as well as the endogenous endorphins.

"Again, Cassandra, I shall drive my heal into your soft pink toes," murmurs the scientist. This being done, she screams forth a number, and that number is Five.

Could she have been merely responding to what was expected of her? Perhaps her previous experience with pain killers had conditioned in her a response to release endorphins.

But perhaps, sweet Cassandra's belief effected the release of endorphins, which then inhibited her pain sense. And so when she was given Naloxone, those endorphins were blocked. Could it be, then, that belief is realized chemically?