Your blood is poisonous to your own body. You must take regular doses of antitoxin to resist the poison's effects.
Drawback
If you go 24 hours without taking a dose of antitoxin, you must make a successful Fortitude save (DC 15) or take 1d4 points of Constitution damage. Taking a dose of antitoxin cures the ability damage caused by your poisonous blood at a rate of 1 point of damage per round. Ability damage caused by your poisonous blood cannot be restored through natural or magical healing.
As an unexpected benefit, creatures that feed off of blood, such as vampires and stirges, gain no sustenance from feeding on you. You still take damage from their blood drain attacks.
Special
A casting of the neutralize poison spell or similar magic can take the place of a daily dose of antitoxin.