Say you were a clever villain holed up in a dungeon where the only obvious way of entering was to swim quite some distance under water. Not to be put off by such obstacles, the heroes simply spend water breathing scrolls, potions or spells and start swimming.

But, being an insidious villain, you have another trick up your sleeve. When the heroes are half way through, an alarm goes off and an intricate rigging of pipes start mixing poison into the water in several places.

My question is this: Would an inhaled poison affect the water breathing heroes, using the 10-foot cube per dose rule?

I'm figuring a contact poison would not work unless they were actually swimming in a fifty-fifty (and very expensive) mixture?

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    \$\begingroup\$ I doubt this situation is covered in the rules. Are you looking for a rules-as-written answer or something to use as a suitable house-rule? \$\endgroup\$
    – Colin D
    Aug 6, 2013 at 20:37
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    \$\begingroup\$ This is a pretty narrow case. It depends on the poison, primarily. If you feel that the poison works by contact or inhalation underwater, then it does. \$\endgroup\$
    – user8248
    Aug 6, 2013 at 21:14
  • \$\begingroup\$ I absolutely agree this is a narrow case. But I often find the narrow ones interesting. I'm looking for any reasonable answer, the closer to the RAW the better. \$\endgroup\$
    – thomax
    Aug 6, 2013 at 21:18

1 Answer 1


tldr; You could say that ingested poisons work in water similarly to the way inhaled poisons work on land.

A very interesting question. I think to start we have to look at how an inhaled poison would work in the natural environment. The particles of the poison diffuse through the air until they are inhaled by the unfortunate victim. For this to occur two things have to happen.

  1. The poison has to diffuse through the air.
  2. The diffused poison has to be inhaled.

Now if we look at what happens with water breathing and any sort of "diffusion" through water. With breathing water the character can breath the oxygen in the water, similar to how any fish does, except magically. So for them to be poisoned underwater it's realistic to expect a poison to be in the water they are breathing.

Whether an inhaled poison could mix with the water or not, I am unsure of this, but think it unlikely. And as you mentioned a contact poison would need to be very concentrated to make it work in a large body of water. However it is possible that you could use an ingested poison in a very similar way in water to an inhaled poison in air.

Whilst this is definitely unofficial, if this were to come up in my game I would rule that ingested poisons diffuse through water like inhaled poisons diffuse through air, and as such any player who swims through, and breaths in, the poison is subject to it's effects. I would also rule that because it is being breathed in, rather than ingested, the time for it to take effect is reduced to instant, or mere rounds (as per your need), as it enters the blood stream much faster.

To be fair, you could limit the poisons that could be used to powders and liquids, as they are the only things that could diffuse through water.

  • \$\begingroup\$ Thanks for the sophisticated answer! You're basically saying that ingested poisons are the underwater equivalent of inhaled poisons, right? I hadn't thought of that! It would definitely be easier to mix a liquid poison into water, than to bubble an inhaled poison (which I expect to be a gas) through the water in an attempt at diffusion. The villain expresses gratitude for your insight, and says you will hear from her regarding an offer of henchmanship :) \$\endgroup\$
    – thomax
    Aug 7, 2013 at 7:28

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