This all changed with the discovery of endogenous opioids (that is, opioids that are developed naturally within the body). Of these morphinelike substances, the most common are the endorphins (a term which is an abbreviation of endogenous morphines). Common situations in which endorphins are produced include childbirth and running the last few miles of a marathon.
Opiates like morphine and heroin reduce the feeling of pain because they mimic the action of substances the body produces on its own to control pain. These drugs bind these same receptors and, at low doses, produce similar effects. However, when injected in large doses, these drugs produce the opposite of pain — a "high" — and are addictive. The drug naloxone antagonizes the effects of these opioids and is often given to addicts to reverse the effects of heroin they have injected.The existence of endorphins also explains another mystery of pain management, the placebo effect. The placebo effect occurs when patients are given a substance that itself has no pain-blocking potential but, because the patients believe they have been given a real drug that will alleviate the pain, find that the pain is actually alleviated.
Although the placebo effect is robust and common, those in the medical community tend to dismiss it as being "psychological," that is, not based on any physiologically demonstrable or quantifiable basis. However, it turns out that the drug naloxone not only reduces the effects of opioids, such as heroin, but it also reduces the placebo effect.
What this means is that the placebo effect isn't just psychological; it actually has a physiological component, involving the cognitive stimulation, from belief, of the body's internal endorphin production that objectively and measurably reduces pain by binding the endorphin receptors.