Narcan or Naloxone is an overdose reversal drug which saves many lives per day. If someone is possibly overdosing, one spray of this may quickly allow the victim to become conscious. This drug works by practically kicking off the opioids from the opioid receptors. It works so well because it is chemically very similar to the common opioid, morphine. This also means it has the same morphinan scaffold as most other opioids.
The only difference between this and other opioids are the fact that Naloxone includes a ketone group (C=O at position 6) instead of a hydroxyl group, it also has a hydroxyl group at position 14 and has an allyl group on the nitrogen (N-allyl substitution).