This is a special perk that can only be obtained from Dr. Troy in Vault City with a Doctor skill of at least 75%, and only after having received the Vault City Training perk. After receiving the Vault City Training perk, upon going to Vault 8 to treat poisoning or radiation, this perk can be asked for.
As with Vault City Training, this is not a regular perk and can't be obtained by leveling up.
This perk appears only in Fallout 2.