Lifegiver

Livegiver or Life Giver is a Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout 4, Fallout Tactics, J.E. Sawyer's Fallout Role-Playing Game, and Lionheart perk.

Fallout, Fallout 2, Fallout Tactics, and J.E. Sawyer's Fallout Role-Playing Game
Every time the player character gains a level, they gain an additional four Hit Points for each rank of this perk. With two ranks of Lifegiver, that's +8 HP each level, which is more than 10 points in Endurance would give you.

Although it's not clear from the description, Lifegiver grants +4 HP for every level gained, including the level when the perk was taken.

Fallout 3, Fallout: New Vegas
You gain an additional 30 Hit Points.

This perk also gives the player character unique dialogue choices when speaking with some specific characters, like Tree Father Birch in Oasis.

Bugs
In Fallout: New Vegas, the health that Lifegiver grants is excluded from the 50% calculation for Ferocious Loyalty.

Fallout 4
This is best as an early-game perk. The math is such that, once one reaches Level 32, they would have been better off just putting 1 more point into Endurance than in getting a level in the Lifegiver perk. However, the benefits of +20 Health early game can be considerable.

The health regeneration gained from the third rank grants the player 0.5% health every second. However, health is not regenerated whilst in combat (i.e. when enemies are nearby.) The health regeneration is fast enough (out of combat) to be a real convenience.

Lionheart
With each rank of this perk, you gain an additional 6 hit points.

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