Constitution modifiers are applied 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.
- With Steadfast Determination, your Constitution bonus is converted into Will save throws only if the Wisdom score is lower than the Constitution score.
If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character's hit points will retroactively increase or decrease accordingly.