Opioids
Opioids are a family of drugs consisting of those derived from opium and of those synthesized in a lab to emulate the effects of opium.
The classification is more determined by the effects on the brain than to the specific chemistry of the substance. For example Methadone is a synthetic opioid. Chemically it is very different than heroin (another opioid); however, the effects on the brain are similar enough for both to be termed opioids, both cause a response at the opioid receptor.
Endorphins are opioids that occur naturally in the body. When humans experience feelings of joy, endogenous opioids are released which cause feelings of happiness and reduce pain. In a healthy human the amounts of endogenous opioids are regulated so that the person never becomes too euphoric or is in too much pain. When external opioids are introduced, the same feelings are experienced, however, at a greater level than can be experienced naturally.
Early on, people found opium had these properties, and it was used for medication for almost any condition found unpleasant. It was discovered that there could always be an appropriate dose that would alleviate any degree of suffering, at least for a short time. This miracle plant was often referred to in ancient literature as “plant of joy” and later as “God’s own medicine”.