Narcotic analgesics are also referred to as opioids. By strict definition, opioids refer exclusively to natural opioid alkaloids and their semi-synthetic derivatives, while natural or synthetic substances that bind to opioid receptors and produce varying degrees of agonistic effects are collectively referred to as opioid-like substances in clinical practice. Opioids refer to the latex-like colloid extracted from the immature poppy plant and left to dry as a solid substance, commonly known as opium or opium, which is now also referred to as a drug. Because opioid receptors exist in the central nervous system associated with pain, and opioids specifically agonize opioid receptors, they can produce analgesic effects. Opioids are exogenous substances that bind specifically to opioid receptors and produce morphine-like effects, including natural opioids and synthetic opioids. At the same time, opioids are highly susceptible to addiction and dependence, and are therefore classified as controlled substances.