At 11th level, monks gain immunity to poison. The Poison Healer feat lets you restore hit points when you "succeed on a Fortitude save against a poison". Can monks chug poison to heal themselves, or do they not actually make the saving throw?
(There seem to be similar questions for other editions, but I can't find one specifically for 3.5e)
This simply isn’t defined anywhere in the rules. There is no technical definition of “immunity,” and it is very likely that the original authors never even considered the question, since in Core, it would never make a difference. (And, truly, even outside Core, it is very, very rare for it to matter anyway.)
Trying to read between the lines in the rules for poisons, saving throws, and so on, is a mistake: none of those address immunity explicitly, and we know immunity is making some kind of exception to those rules. Where the exception comes in and what it applies to makes all the difference to this question, and the rules have zero guidance for us on that.
The one thing we do have is neutralize poison (with thanks to Peregrin Took for pointing it out), which says
The creature is immune to any poison [and] the creature need not make any saves against poison effects applied to it during the length of the spell.
(SRD → Spells → Neutralize Poison)
To me (and not to Peregrin), “need not” is quite distinct from “does not” or “cannot,” and directly implies that making the saving throw is an option. However, this rule only applies to immunity from the neutralize poison spell, because that’s where the rule is found. It’s a specific case that could be an exception—maybe every other type of immunity works differently and neutralize poison is just offering an extra, special benefit of also having the option of skipping the saving throw (or having the effect of always skipping the saving throw, whether you class that as a “benefit” or not). Maybe. Or otherwise, perhaps, it is an example of what was intended for immunity in general, and we should apply the same rule everywhere. Unfortunately, this is still very unclear, and only helps us somewhat.
So you are simply going to have ask your DM, or if you are the DM, you are simply going to have to make a ruling.
Saving Throws (PHB p.136):
Generally, when you are subject to an unusual or magical attack, you get a saving throw to avoid or reduce the effect.
If you're immune to poison, there's nothing for you to avoid or reduce against a poison effect, it does nothing to you.
Also, making a saving throw would imply throwing dice, which would mean you could fail:
Saving Throws - Automatic Failures and Successes (PHB p.136):
A natural 1 (the d20 comes up 1) on a saving throw is always a failure.
Which would be absurd if you're immune.
Also, there is the core mechanic of the D&D 3.5 system:
Playing The Game -> The Core Mechanic (PHB p.4)
Whenever you attempt an action that has some chance of failure, you roll a twenty-sided die (d20).
Since you're immune, you don't have any chance of failure against a poison effect.
Rules as written, this works. Whether or not this is intended is up to your GM.
From the SRD:
When a character takes damage from an attack with a poisoned weapon, touches an item smeared with contact poison, consumes poisoned food or drink, or is otherwise poisoned, he must make a Fortitude saving throw. If he fails, he takes the poison’s initial damage (usually ability damage). Even if he succeeds, he typically faces more damage 1 minute later, which he can also avoid with a successful Fortitude saving throw.
This is the description of the poison ability. At no point does it mention whether the victim would be susceptible to the effects.
It's worth noting that this technique is pretty terrible. The cheapest (non-inury)poison listed on the SRD is oil of taggit for 90gp. Assuming 16 con and the ability to reliably make the saving throws, that's 90gp for 6 healing, 3 points of which take a minute.
For this you need 2 feats- both of which are useless outside this. You also need to invest in poison immunity, which can be done more easily than taking monk levels (be a warforged for example). Finally, you need access to poison which is specifically mentioned to be difficult and illegal.
Alternatively, for 50gp, you could buy a potion of cure light wounds.
