Spider Venom extracted from any Spider cannot be applied to any type of weapon. There is no DEX roll involved either. The message you receive is: "Nothing happens."
It cannot be used to poison food either (though that's not supposed to be the case).
To bump up this, the item I received from harvesting a racing spider was "Spider venom (potent)" when in inventory allthough it said "Acquired item: Centipede venom, weak" at first.
With the use poison feat, the game mechanics message is "[Name] coats his weapon with poison!" but then there's a message in yellow text saying "* Nothing happens *"
Are there some poisons (not including poisons for food/drinks) that are working correctly on weapons?
I don't know what's up with this.
Last time I used a poison on a weapon (not the Spider Venom), it worked (it was one of the poisons that drop or can be picked in the kobolds quest).
And I saw the post doesn't address it, but the Spider Venom also makes the same "* Nothing happens *" message when applying it to potions. All were done with the use poison feat.
The custom weapon poisons aren't working.
The weapon poisons that are vanilla with NWN are working.
Just a note that this is still not working. I suspect it's what Frenzied posted. Those I find on quests and the like work fine, the ones I take off of spiders give me the *Nothing happens*.