Swallow

The Witcher

"There is no bird more beautiful than the swallow, the harbinger of spring. Even the dark mages who developed the formula for witchers' potions appreciated the charm of this bird, lending its name to the potion that accelerates regeneration of a mutated organism."

Swallow accelerates the regeneration of Vitality.

Source

The Book of the Swallow
Geralt receives this potion from Vesemir in the Prologue during the Defending Kaer Morhen quest.
There are three swallow potions in the wardrobe in the witchers' laboratory in Kaer Morhen in the Prologue.
Two more in a crate in the Evening hall on the second floor of Kaer Morhen.

The Witcher 2: Assassins of Kings

Geralt begins the game with four Swallow potions as well as the formula.

Journal Entry

Symbolizing spring and rejuvenation, the swallow lent its name to this potion that accelerates the rate at which wounds scab over and heal. As a universal brew, it is good on many occasions and additionally produces no side effects.

The witcher can use the Swallow potion to balance out some of the undesirable effects of potions like Thunderbolt, Maribor Forest, Tiara and Stammelford's Philtre.

The potion increase Toxicity slightly when imbibed.

It is recommended before long fights. Slow but consistent Vitality regeneration will provide a significant advantage over foes. Witchers unsure of the dangers they might face in a given battle should take Swallow.