Opioid, opiate and narcotic mean different things, and tolerance is uneven
Your own body makes opioids: endorphins are one family of them. The word covers anything, natural or lab-made, that binds the brain's opioid receptors, from poppy-derived morphine to fully synthetic fentanyl. Opiate, strictly, means only the alkaloids found in the opium poppy's resin, and narcotic originally meant anything that numbs.
Opioid receptors sit mainly in the brain and spinal cord, the peripheral nerves and the gut, which explains the drugs' mix of effects. Some compounds never cross from the blood into the brain. Loperamide, an over-the-counter anti-diarrhoea medicine, acts only on receptors in the intestinal wall, and the flip side of that gut effect is the constipation that follows longer use of stronger opioids. The antagonist naloxone does the reverse of the others, knocking opioids off their receptors, which is why it is used to reverse overdoses. Carfentanil, at the extreme end of potency, is approved only for animals.
Tolerance does not develop evenly. The body adapts relatively quickly to the pain-relieving effect and some physical side effects, but only slowly to effects on mood and breathing, and some researchers argue it never develops to constipation or pinpoint pupils. Tolerance and physical dependence, meaning withdrawal symptoms when a drug stops abruptly, are normal adaptations and do not by themselves indicate addiction. Addiction is a separate problem, typically driven by escalating recreational use for the euphoria.
The medical picture has shifted. Opioids are highly effective for severe short-term pain, such as after surgery, and important in palliative care for cancer, yet a 2018 report found that starting them for acute pain carries a clear risk of prolonged use. Guidelines now judge their risks likely to outweigh benefits for most non-cancer chronic pain, including headaches and back pain. Codeine, once considered the gold standard for cough, performed no better than placebo in some trials involving children.
The scale of harm is large. Illicit use rose from an estimated 28 to 38 million people in 2013 to 60 million by 2021, and about 69,000 people worldwide die of opioid overdose each year. Growing use of opioids for non-cancer chronic pain from the late 20th century fed the opioid epidemic, while fear of overprescribing has, conversely, been blamed for pain going untreated.
Source: Opioid
