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You apply your character’s Constitution modifier to:
- Each roll of a Hit Die (though a penalty can never drop a result below 1—that is, a character always gains at least 1 hit point each time he or she advances in level).
- Fortitude saving throws, for resisting poison and similar threats.
- Concentration checks. Concentration is a skill, important to spellcasters, that has Constitution as its key ability.
Any living creature has at least 1 point of Constitution. A creature with no Constitution has no body or no metabolism. It is immune to any effect that requires a Fortitude save unless the effect works on objects or is harmless. The creature is also immune to ability damage, ability drain, and energy drain, and automatically fails Constitution checks. A creature with no Constitution cannot tire and thus can run indefinitely without tiring (unless the creature’s description says it cannot run).