Fast Metabolism

Fast Metabolism is a Fallout, Fallout 2, Fallout Tactics and Lionheart Trait as well as a Fallout 3 and a Fallout: New Vegas Perk. In Van Buren it was renamed to Increased Metabolism.

Fallout, Fallout 2, Fallout Tactics, Van Buren
This trait adds +2 to your Healing Rate, but your radiation and poison resistances start at 0%. Unavailable to ghouls and robots (Fallout Tactics).

Lionheart
You have an increased Healing Rate, but your disease and poison resistances start at -10%. Feralkins cannot take this trait.

J.E. Sawyer's Fallout RPG
Your metabolic rate is increased. This means that you are much less resistant to radiation and poison, but your body heals faster. -10% to radiation and poison resistance, +2 to healing rate.

Fallout 3, Fallout: New Vegas
With the Fast Metabolism perk, the player gains a 20% bonus when using Stimpaks. In Hardcore mode, the multiplier increases the amount of healing per second making stimpacks an even better in-combat healing option. Players outside of Hardcore mode may be better served by simply applying multiple stimpacks from their Pip-boy rather than investing in the perk.