Rat tonic is a healing potion for specifically intended to heal rats. It can also be used to treat the symptoms of ageing in rats. It is sold in a small red bottle at the Magical Menagerie in Diagon Alley, and presumably at similar locations elsewhere.
It is also the case that excessive use of Rat tonic can result in the rat drinking it becoming engorged, enlarged and even deformed. It is recommended that the directions on the bottle be followed to the letter.
Related drugs: Confusing Concoction, Shrinking Solution and Wolfsbane Potion.