From Brotato Wiki
Regeneration Potion is a unique item. Only 1 of each unique item can be equipped, and it will then no longer be offered in the Shop, and no longer drop from loot crates.
Simliar to the Doctor's ability, the Regeneration Potion makes your HP Regeneration heal +2 HP per tick of healing, rather than doubling the rate of ticks. It also stacks with the Doctor's ability to heal +4 HP per tick.