Are there mechanical disadvantages to being unable to breathe for a handful of rounds, before you are actually suffocating?

Question

Several monsters (for example, the Darkmantle) have abilities that cause their victims to be unable to breathe. However, when this happens in-game my characters just shrug and say, "whatever, I can hold my breath for 4 minutes." The battle is long over before suffocation becomes an issue. Similarly for situations like running into a AOE (such as a Stinking Cloud where breathing would be ill-advised.

Are there any mechanical disadvantages to being unable to breathe, before you get to the point of suffocating?

I know the rules for Suffocating from PHB p. 183 -- the question is, is there any mechanical penalty to not breathing during the minutes that you can hold your breath.

It's an interesting question as to whether, for example, the Darkmantle attack allows you to "hold your breath" or moves you directly to "choking" but that's really a separate question. What I am really asking here is, if you are holding your breath, as per the game rules on PHB p. 183, can you fight at 100% efficiency? Or should, say, disdvantage apply to attacks rolls, movement rate be reduced, etc.?

Note: Jeremy Crawford has ruled that you can hold your breath.

Q: If you can't breathe due to a monster effect (ie Darkmantle) are you immediately suffocating? Or have Con Mod+1 mins?

A: You can hold your breath.

(See Looking for play tested house rules for combat while holding your breath for a follow-on question about whether anyone has any good house rules for this situation.)

Answer 1

Depends if you hold your breath

The player's handbook p.183 gives rules for suffocating. In brief a creature holding its breath can last for minutes but a creature that runs out of breath or is choking lasts a number of rounds equal to their constitution modifier before dropping to 0 hp.

The question then is whether a PC is holding their breath when being suffocated by, say, a darkmantle. The darkmantle monster entry says a creature engulfed is, "unable to breath", but gives us no further information. This seems to be up to the DM to rule if a given character was able to begin holding their breath, and so lasts minutes, or if the suffocation begins suddenly and they can only last a few rounds.

Personally I'd go with effects such as this happening too suddenly to hold your breath since it keeps the effects relevant in an average battle.

RAW, you can hold your breath because Jeremy Crawford says so

If we're counting JC tweets as RAW than all that matters is JC says you can. As you explained in your question this means that the suffocation part of fighting a darkmantle is unlikely to ever come up in a normal battle, and I'd say this takes away what makes the darkmantle an interesting monster. Just another strange part of 5e RAW.

Answer 2

As others have pointed out, PHB p183 gives rules for suffocating:

Suffocating
A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).
  When a creature runs out of breath, it can survive for a number of rounds equal to its Constitution modifier (minimum 1 round). At the start of its next turn, it drops to 0 hit points and is dying.

It is perfectly reasonable to modify this depending on circumstances. It is dramatically different to hold your breath just sitting on the couch vs. holding your breath after a couple of minutes of combat, or even after a few minutes of moderate exercise. Try climbing a couple flights of stairs and then hold your breath at the top.

At a minimum, it's reasonable to impose disadvantage at some point as a character is suffocating.

The PHB and DMG support this:

PHB p171:

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.

DMG p239:

Advantage and disadvantage are among the most useful tools in your DM's toolbox. They reflect temporary circumstances that might affect the chances of a character succeeding or failing at a task.

Answer 3

The rules for suffocation are on p.183 of the PHB; eventually you will become unconscious. However, until that happens, there are no in game effects.

Answer 4

Based on the requirements for certain tasks in game, there are some mechanical things that a character cannot do while being unable to breathe:

• You can't use your sense of smell (so any perception checks depending on smell would be failed).
• You can't speak.
• You can't cast spells requiring verbal components.