Dragon Nip is a substance introduced in Book 7 of the How to Train Your Dragon Book series.
Dragon-nip is a yellow, bubbly substance harvested from the sweat gland of the Loafer Dragon, that can effect both humans and dragons.
Dragon Nip effects the behavior of dragons, who seem to seek it out. It causes drunken behavior and eventually sleep. In Dragons it parallels alcoholic beverages in humans.
In humans, it can cause drowsiness and sleep.
In How to Ride a Dragon's Storm, While on The American Dream 2, Toothless and Stormfly steal some dragon nip from the kitchen, and drink it, becoming drunk. This, however, gives Hiccup the idea to lace the Hysteric's food with dragon nip, which causes them all to fall asleep. Then he helps the Wanderers, who were being kept as slaves, escape on landing boats.