Why do opioids and opiates work so well?
It all starts with the poppy, somewhat ironically known as both a symbol of medicinal power and death. As psychiatrist Anna Fels has noted, prescribed uses of the poppy plant (especially opium) and its descendents have been around for millennia -- opiates can refer to "close relatives of opium," like codeine, morphine, and heroin, while opioids include synthetic opiates like OxyContin, Percocet, Vicodin, and Dilaudid. Heroin is the only drug among those that's whole-hog illicit in the United States. That's a particularly odd medical quirk, especially since, when abused, OxyContin and Percocet function very similarly to heroin on brain chemistry.
But why do these drugs feel so damn good?
The human brain is essentially wired to respond to opioids, holding a concentrated collection of mu-opioid receptors ready to make you feel better at a moment's notice. Receptors send and receive messages in the brain, allowing neurons to communicate with one another; opioids send the message, "You are relieving your pain now."
These receptors are present in several areas of the brain, namely the regions that manage perception of pain AND the regions that fire off feel-good sensations. Yup, the latter are the parts of your brain that tell you: "Whoa, you're high, buddy!" Opioid medications don't treat the source of the pain, though, which means they don't target muscular injuries, nerve distress, or bone disease. Instead, opioids funnel straight to the brain, the place where we process what the heck's going on elsewhere in the body, in a sense tricking the brain into not recognizing the source of discomfort. This is important.