Part of an ongoing series of 5e (2014) Rules notes.  See the end of the post for notes on 5.5e (2024) rules.

The first time I DMed this, I did it wrong. Which, given it was an NPC I had thought I had well in hand, is not a cool move on my part.

So here it is, done right.

Here’s the core of the spell’s effect:

Each creature that is completely within the cloud at the start of its turn must make a Constitution Saving Throw against Poison. On a failed save, the creature spends its Action that turn retching and reeling.

When I first played with this, I ruled that this still allowed Movement (since that isn’t mentioned), but, just as anything that takes away your Action also takes way your Bonus Action, the only thing you could do was retching and reeling.

But that’s not what it says. The Stinking Cloud doesn’t take away your Action, it dictates your Action (retching and reeling). I.e., your Action is set, but you still have your Bonus Action (and Reaction, for that matter).

Or, as the Sage Advice Compendium puts it:

The stinking cloud spell says that a creature wastes its Action on a failed Save. So can it still use a Move or a Bonus Action or a Reaction?

Correct. The gas doesn’t immobilize a creature or prevent it from acting altogether, but the effect of the spell does limit what it can accomplish while the cloud lingers.

Movement is a bit problematic, of course. The area covered by Stinking Cloud  is Heavily Obscured.

A heavily obscured area–such as darkness, opaque fog, or dense foliage–blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

Or, presumably, out of that area. Blinded, in turn:

A blinded creature can’t see and automatically fails any Ability check that requires sight.

Attack rolls against the creature have Advantage, and the creature’s attack rolls have Disadvantage.

Note the offsetting penalties — trying to Attack someone inside the cloud has to deal with Heavily Obscured conditions, and so is at Disadvantage (as though Blinded). But the target is, themselves, Blind to the attack, putting them at a Disadvantage. That makes, even without all the loud retching sounds, attacks on a figure within a Stinking Cloud even money. (A figure inside the cloud can’t Attack if they fail their Save, except through a Bonus Action, but with that Bonus Action, or if they make the Save, theoretically, they are also a wash to attack a target outside the cloud, unless that target is using Stealth or a Dodge or something of that sort.)

I might House Rule that, combined with the Retching and Reeling, being Blinded in such a circumstance would lead to disorientation — perhaps another Save (Intelligence?) to move in a desired direction?

As a final note, the rules say “completely within the cloud” for the nausea effect. So if you are playing on a grid, and are using a true circle for your template (physically or on a VTT), any one in a partially covered circle isn’t affected. Which is why I prefer to have a template that fills in complete boxes on the grid, to avoid the ambiguity.

Any changes here in 5.5e?

The rules in 5.5e (2024) contain several changes to Stinking Cloud:

You create a 20-foot-radius Sphere of yellow, nauseating gas centered on a point within range. The cloud is Heavily Obscured. The cloud lingers in the air for the duration or until a strong wind (such as the one created by Gust of Wind) disperses it.

Two changes here: first, there’s no mention of the cloud spreading around corners (though that seems like something still to naturally consider; a cloud of gas doesn’t respect cover). Second, only a “strong wind” now disperses it, but apparently that’s instantly, not after a number of rounds.

Each creature that starts its turn in the Sphere must succeed on a Constitution Saving Throw or have the Poisoned condition until the end of the current turn. While Poisoned in this way, the creature can’t take an Action or a Bonus Action.

The requirement to be completely within the cloud to be affected is gone, which increases the radius a bit and also helps with Large creatures. The “retching and reeling” color text is, alas, gone, but we now have the Poisoned condition explicitly called out and Bonus Actions have now been sealed off, too.

Also, there’s no mention of the 5e “creatures that don’t need to breathe or are immune to poison” auto-saving. The latter would still (kind of by definition) be immune (suffering only from the Heavy Obscurement and effective Blindness), but the former are, like tear gas victims, deemed Poisoned (if they fail their Save) by contact with the gas, not just inhalation.

Overall, an increase in impact for using the spell.