Nectar

In Greek mythology, Nectar is a drink consumed by the Gods. It gives immortality to whoever drinks it. It is closely related to Ambrosia (for healing purposes). Nectar can be drunk by wounded demigods.

Percy Jackson and the Olympians
In Percy Jackson and the Olympians, nectar is used as a healing drink to be used when someone is injured. Demigods can drink it, but if they have too much of it they are in danger of being incinerated. Humans can't drink it at all without being incinerated. It has been implied that nectar tastes like anything the drinker likes (favorite food or drink). This is shown in The Lightning Thief, when after recovering from the battle with the Minotaur, Percy is surprised to find that the nectar tastes like his mother's blue chocolate chip cookies. Nectar is closely related to Ambrosia.

Film
Nectar does not appear in the Lightning Thief film, although Percy is given nectar by Annabeth in a deleted scene. It's flavor is never mentioned although Percy states that it's 'amazing'. Also, Annabeth mistakenly calls it the 'food' of the gods.

Consumption
Nectar is the drink of the gods. It is also used for healing by demigods. If a mortal eats it, they would be incinerated. A demigod can eat a little, but taking an overdose could make them feverish. A huge overdose would cause them to incinerate as a mortal would.