An opioid is a chemical substance that has a morphine-like action in the body. The main use is for pain relief. These agents work by binding to opioid receptors, which are found principally in the central nervous system and the gastrointestinal tract. The receptors in these two organ systems mediate both the beneficial effects, and the undesirable side effects. There are a number of broad classes of opioids:

• natural opiates, alkaloids contained in the resin of the opium poppy including morphine, codeine and thebaine, but not papaverine and noscapine which have a different mechanism of action;
• semi-synthetic opiates, created from the natural opioids, such as hydromorphone, hydrocodone, oxycodone, oxymorphone, desomorphine, diacetylmorphine (Heroin), nicomorphine, dipropanoylmorphine, benzylmorphine and ethylmorphine;
• fully synthetic opioids, such as fentanyl, pethidine, methadone, tramadol and propoxyphene;
• endogenous opioid peptides, produced naturally in the body, such as endorphins, enkephalins, dynorphins, and endomorphins.

Although the term opiate is often used as a synonym for opioid, it is more properly limited to the natural opium alkaloids and the semi-synthetics derived from them.

Some minor opium alkaloids and various substances with opioid action are also found elsewhere in nature, including alkaloids present in the Kratom, Corydalis, and Salvia plants and some species of poppy aside from Papaver somniferum, and there are strains which produce copious amounts of thebaine, an important raw material for making many semi-synthetic and synthetic opioids. Of all of the more than 120 poppy species, only two produce morphine.

It has been discovered that the human body, as well as those of some other animals, naturally produce small amounts of morphine and codeine and possibly some of their simpler derivatives like heroin and dihydromorphine, in addition to the well known endogenous opioids. Some bacteria are capable of producing some semi-synthetic opioids such as hydromorphone and hydrocodone when living in a solution containing morphine or codeine respectively.

Amongst analgesics are a small number of agents which act on the central nervous system but not on the opioid receptor system and therefore have none of the other (narcotic) qualities of opioids although they may produce euphoria by relieving pain -- a euphoria that, because of the way it is produced, does not form the basis of habituation, physical dependence, or addiction. Foremost amongst these are nefopam, orphenadrine, and perhaps phenyltoloxamine and/or some other antihistamines. The remainder of analgesics work peripherally. Research is starting to show that morphine and related drugs may indeed have peripheral effects as well, such as morphine gel working on burns, but peripherally-acting analgesics include aspirin, ibuprofen and the like. Paracetamol is predominantly a centrally acting analgesic (non-narcotic) which mediates its effect by action on descending serotonergic (5-hydroxy triptaminergic) pathways, to increase 5-HT release (which inhibits release of pain mediators). It also decreases cyclo-oxygenase activity.