Hypersmurf
Moderatarrrrh...
The only Hps it would've taken are those bestowed by Con hitpoints, right?
Well, your current hit points are always your total rolled points per die, plus your Con bonus x Hit Dice, less any damage sustained.
The example I'm using is a Young Adult Blue Dragon. 19 Con, 18 hit dice, 189 hit points average.
That's a Con bonus of +4 : 4 points per hit die. So it's hit points could be considered 117, + 72 bonus points.
If it sustains two points of Con damage, its bonus drops to 3 points per die. Its hit point total is now 117 + 54, or 171.
If its Con drops to 6, then its Con bonus is -2. Its hit point total is now 117 - 36, or 81.
Any damage sustained is subtracted from that. So if the dragon has taken 100 hit points of damage, and then gets its Con drained to 6, then its current hit points are 81 - 100, or negative 19. Dead, in other words.
The potential problem for DMs to keep track of, if they can be bothered, is that no hit die can be worth less than 1 hit point.
So with a Con modifier of -4 (say a 2 Constitution), a roll of 6 is worth 2, a roll of 5 is worth 1, and a roll of one through 4 is also worth 1. But figuring out what the dragon rolled for each d12 can be a headache

-Hyp.