From Brotato Wiki
+3 HP Regeneration
Regeneration Potion is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Regeneration Potion is a unique item. Unique items can only be bought or found once per run.
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