From Tolkien Gateway
John Howe - Entdraught

Ent-draughts were the mysterious and potent waters consumed by the Ents which Treebeard shared with the hobbits in the Wellinghall.

Though little is known about them in detail, they seem to have been made from river-water, but with unique and subtle flavours reflecting the Ents' association with trees and forests. The first time Merry and Pippin imbibed an ent-draught it reminded them of the smell of a distant wood brought from afar by a cool night breeze. The second time they drank the flavour was earthier and richer, more food-like.[1]

Their power of growth was most evident when they were drunk by beings other than Ents: the Ent-draughts taken by Merry and Pippin added three inches or more to their heights,[2] making them the tallest Hobbits in the history of the Shire.[3]