Negative CON hit points question

[Gevurah]

If my Animal Companion (or any character) has a CON of 9 (-1) and at some level it manages to get it up to 10 (0), does it retroactively gain HP from previous levels?

[Pizza Lord]

Yes. He gains or loses extra hit points from Constitution retroactively per HD. This makes Con damage a dangerous effect, though you can't go below a minimum of 1 hit point per hit dice from this loss itself.

Abilities wrote:

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.

[Warped Savant]

I'm not at home right now so I'll have to find it later, but I'm sure that when you lose Con your HPs drop for every 2 Con you lose, not based on what your new score is. (Which seems weird, I know. That's why I remember it. I was so surprised it was that way rather than based on new score.)

[Ciaran Barnes]

Warped Savant, that would be true for ability score damage but not when a creature’s base ability score is 9.

[Thewms]

The following information is not official in terms of general campaign usage. It is copied from the Pathfinder Society Organized Play FAQ section because it might be helpful information for a player or GM in adjudicating common problems or questions. Usage is up to the GM of your game.

How do I determine hit points for my animal companion?

Animal companions receive average hit points per hit die. For d8, the average is 4.5. Multiply 4.5 times the number of hit dice your animal companion has and round down. Recalculate hit points for your companion each time it gains additional hit dice. After its hit points are calculated, add in appropriate bonuses from its Constitution modifier, feats, and so on.

[Gevurah]

Awesome, thank you guys. I was precisely going to use the Animal Companion in a PFS game, so your response is much appreciated.

[Warped Savant]

Ciaran Barnes -- Right! Sorry, I was distracted and thinking that the question was about Con drain. (That'll teach me to answer questions early in the morning!)