From Brotato Wiki
+3 HP Regeneration
Item
Regeneration Potion is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Unique
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.
Regeneration Potion
The Regeneration Potion gives you a special ability to double your HP Regeneration when you are below half of your Max HP.
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.
The Regeneration Potion is a very powerful item on builds that relies on HP Regeneration for healing, commonly seen on characters such as Bull, Doctor, Lich, or Masochist