A Potion heals 20 HP of a Pokémon. Has no effect on a fainted Pokémon. There is one in your PC at the start of the game, except in Generation IV. It is 300 Poké dollars at Poké Marts.
Other locations
R/B/Y/FR/LG: Poké Mart worker on Route 1.
R/S/E: Poké Mart worker in Odale Town
D/P/Pt: Poké Mart worker on Route 201
Description
The potion looks like a blue vase with a sky blue foam bubbling out. Its not known whether it is drunk or poured on the wound.