From DDO wiki
You apply your character’s Constitution modifier to:
- The number of Hit points you get each level.
- Fortitude saving throws, for resisting poison and similar threats.
- Concentration checks. Concentration is a skill, important to spell casters, that has Constitution as its key ability.
- A monk's stable ki level is also equal to his or her total Concentration skill.
Raising Constitution Scores
You can raise your Constitution score from different sources, among them:
- During character creation, you can add up to 10 points to your basic Constitution score, which means:
- At the 4th, 8th, 12th, 16th, 20th and 24th levels, you are allowed to raise a single ability by 1 point. If invested in Constitution and the ability score moves to the even number, this will result in an increase in character HP.
- Inherent bonus - A Manual of Bodily Health will raise the Constitution score by the appropriate value. For more information, refer to Tomes.
- Feat Bonus
- Class Enhancements
- Racial Enhancements - Four races allow to raise the Constitution score via enhancements:
- Completionist feat - bonus +2 to all abilities.
- Enchanted Bonus - Unless noted otherwise, bonuses within a single category do not stack.
- Item Enhancement - Items may come with the enchanted bonus of +1 - +8.
- Exceptional Constitution - Some rare items may come with a +1 Exceptional Constitution bonus. (+2 Exceptional bonuses were changed to insightful bonus in Update 14.)
- Insightful Constitution - Other rare items may come with a +2, +3 or +4 Insightful bonus.
- Guild Airship buff shrines - Provides a +1 or +2 Artifact bonus. (Note: Modern ship buffs only provide +2, but +1 or +2 legacy shrines may still be purchased.)
- Yugo Pot bonus - The Yugoloth Favor Potion, Essences of Desire, adds a +2 bonus to constitution for 15 minutes).
- Potion of Health - The House Deneith Collector, Zolar Drexxian, may provide a Potion of Health +1 - +3 alchemical bonus, in exchange for particular collectables.