Rat tonic

A Rat Tonic is a healing potion for rats. Apparently, it is used to treat ageing rats. It is sold in a small red bottle at the Magical Menagerie in Diagon Alley.

When Ron Weasley takes his pet rat Scabbers to the Magical Menagerie because he looks ill, the witch behind the counter suggests Rat Tonic. Before Ron can pay, Scabbers is attacked and chased out of the shop by Crookshanks. Ron and Harry Potter follow. Hermione Granger stays at the shop to buy Crookshanks and the Rat Tonic, which she passes on to Ron