Ant Nectar

Ant nectar is a chem naturally produced by giant ants.

Fallout 3
It is relatively rare since it is only found on few giant ants or in Ant Refuse Piles. A few can be found on the dead body of William Brandice in the Marigold Metro Station. It can also be found by going to Shalebridge and searching the large mound in the same room that you find the large ant egg pile and the Ant Researcher, and checking the mound every few in game days.

Fallout: New Vegas
In Fallout: New Vegas, ant nectar is most commonly found in Ant Refuse Piles or on dead ants. Several Ant Refuse Piles can be found in and around the Ant Mound

If taken in excess, you can get addicted to it. Unfortunately, its addiction appears to be uncureable.(Note:this was fixed in Fallout: New Vegas patch 1.2.0.31x). Doctor Mitchell in Goodsprings, Ada Straus in Novac nor Julie Farkas in the Old Mormon Fort are able to cure it.