The opioid antagonists like naloxone and naltrexone work by blocking the opioid receptors.
Imagine you have a square hole, the receptor, and a rectangular brick, the opioid. Under normal conditions, the rectangle can fit in the square hole in certain directions but it’s always loose and not perfect. Then comes a square brick, the opioid antagonist: because it fits perfectly in the hole, it knocks the rectangle out of the hole and fits in perfectly, locking itself in there and not letting other rectangles enter
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