This is the morphine molecule.
What gave the morphine molecule its immense power, he said, was that it evolved somehow to fit, key in lock, into the receptors that all mammals, especially humans, have in their brains and spines. The so-called mu-opioid receptors—designed to create pleasure sensations when they receive endorphins the body naturally produces—were especially welcoming to the morphine molecule. The receptor combines with endorphins to give us those glowing feelings at, say, the sight of an infant or the feel of a furry puppy. The morphine molecule overwhelms the receptor, creating a far more intense euphoria than anything we come by internally. It also produces drowsiness, constipation, and an end to physical pain.
Tobacco, coca leaves, and other plants had evolved to be pleasurable and addictive to humans, Coop said. But the morphine molecule surpassed them in euphoric intensity. Then it exacted a mighty vengeance when a human dared to stop using it. In withdrawal from the drug, an addict left narcotized numbness and returned to life and to feeling. Numbed addicts were notoriously impotent; in withdrawal they had frequent orgasms as they began to feel again. Humans with the temerity to attempt to withdraw from the morphine molecule were tormented first with excruciating pain that lasted for days. If an addict was always constipated and nodding off, his withdrawals brought ferocious diarrhea and a week of sleeplessness. The morphine molecule resembled a spoiled lover, throwing a tantrum as it left. Junkies I talked to, in fact, said they had an almost constipated tingling when trying to urinate during the end of withdrawal, as if the last of the molecule, now holed up in the kidney, was fighting like hell to keep from being expelled. Like a lover, no other molecule in nature provided such merciful pain relief, then hooked humans so completely, and punished them so mercilessly for wanting their freedom from it.
Quinones, Sam. Dreamland: The True Tale of America’s Opiate Epidemic (pp. 38-39). Bloomsbury Publishing. Kindle Edition.