D&D 3.5e HP Damage and Constitution Damage

Question

Say my character is level 5 and I have 12 constitution (+1 modifier) with 25 total hit Points. What happens if I have 5 points of damage (20 out of 25 total hit points) and then I take 1 point of constitution damage, bringing down to 11 with no modifier? Do I go to 20 out of 20 total hit points, or do I go to 15 out of 20 total hit points?

Similarly: if I have 20 out of 20 total hit points and 1 constitution damage, and the constitution damage heals do I recover to 20 or 25 out of 25 total hit points?

Answer 1

If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly. - From the d20srd. (Thank you Tridus for the rule citation)

With the +1 modifier, you've got +5 HP for being fifth level, but your wounds are still leaving you with -5 hit points off your max: 20 hit points out of a max of 25. Taking con damage to reduce your maximum leaves you with 15/20. Consider this a subtraction of your current damage from your maximum hit points.

Similarly, your constitution raising increases your maximum but doesn't make your wounds worse (give you more damage). Going back up to a +1 modifier gives you five more hit points as a fifth level character, in addition to increasing your maximum to its previous state.

Answer 2

From The Basics – Constitution:

If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly.

Thus a change in Constitution that changes your Max HP changes your Current HP by the same amount.

Answer 3

Remember that how hurt you are is not determined by your HP total, but the proportion between your current HP and your maximum. Characters with 75/75 HP or 6/6 HP are both unharmed, but the 75/75 character is not any healthier than the 6/6 character; he's just a lot harder to kill. When you get hurt, that proportion gets closer to zero. When you get healed, the proportion get closer to 1.

If you keep this in mind, the rest of the question falls into place. When you lose max HP, you also lose current HP, because you're getting hurt: this is what brings the proportion closer to zero. When you gain max HP, you also gain current HP, because you're being healed: this is what brings the proportion closer to 1. It would look really strange if a character's wounds started closing when he took Con damage.