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Poison attacks deal initial damage, such as ability damage or some other effect, to the opponent on a failed Fortitude save. Unless otherwise noted, another saving throw is required 1 minute later (regardless of the first save’s result) to avoid secondary damage. A creature’s descriptive text provides the details.
A creature with a poison attack is immune to its own poison and the poison of others of its kind.
A successful save avoids (negates) the damage.
See also: poisons.