" 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. Aspirin had a limit to the amount of pain it could calm. But the more morphine you took, Coop said, the more pain was dulled. "

― Sam Quinones , Dreamland: The True Tale of America's Opiate Epidemic