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, you gain a 20% Health bonus when using Stimpaks. It is debatable how useful this perk is outside of Hardcore mode. It depends on if you have a high medicine skill. If your medicine skill is lower than 40 you should take this perk. If it is greater than 45 you should not need to take this perk as you already have a good skill with stimpaks. If you are a miser, then you should be able to pay your medical bills at some town or buy stimpaks from one of the vendors in the wastes.