Naloxone
When a person takes opioids, the opioids have to attach to certain receptor sites in the brain in order to work, like a lock in a keyhole.  Once the opioids attach to these opiate receptor sites - like a lock fitting into a keyhole - the opiates start to work.  They kill pain, create euphoria, and make people feel calm and relaxed.  But if a person takes too many opiates, they can also make it impossible to breathe.