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There are many places where game elements interact with and react to damage.

When your current HP drops as a result of your HP maximum dropping, does that count as damage according to RAW?


Example

So, a character with a HP max of 30 and a current HP of 25 gets affected by the part of a mummy's rotting fist which says:

[the target's] hit point maximum decreases by 10 (3d6) for every 24 hours that elapse.

(so this would be long after any initial damage from the rotting fist)

After taking 10 points of HP max decrease from this, the character would then be at a HP max of 20 and a current HP of 20. Thus effectively losing 5 current hit points.


The only thing I've been able to find so far is this tweet by Jeremy Crawford which says:

"Damage" is a game term, involving the loss of hp...

I mean it is not a huge amount to go off of, but the character in my example definitely lost HP (25-20 current HP).

So, I'm looking for further rules and places in the sourcebook that might answer the question one way or another.

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(Most of this answer is copied from my answer to your other question. This is certainly asking a different question, but much of what I said there applies.)

What are Hit Points, and why do they change?

First, let's make sure we know what hit points actually are.

Hit Points

Hit points represent a combination of physical and mental durability, the will to live, and luck. Creatures with more hit points are more difficult to kill. Those with fewer hit points are more fragile.

A creature’s current hit points (usually just called hit points) can be any number from the creature’s hit point maximum down to 0. This number changes frequently as a creature takes damage or receives healing.

Whenever a creature takes damage, that damage is subtracted from its hit points. The loss of hit points has no effect on a creature’s capabilities until the creature drops to 0 hit points.

It's generally established, though I don't know of a rule that makes it perfectly explicit, that if your maximum hit points becomes lower than your current hit points, that your current points becomes your maximum hit points (See "What happens to current hp upon loss of max hp?".) Since due to the definition of "current hit points" it needs to be within the bounds of 0 to your maximum, and your maximum has changed, it makes sense that it has to change as well. But it'd be real silly if it were to change to, say, 0, or to a random number less than the new maximum. Changing to the closest number that's still within the bounds allowed for current hit points makes the most sense, and nobody would really expect it to work differently.

Does this change to your current hit points count as damage?

Your hit points just "becoming" a number is part of what's a bit odd about all this. As you've noted, it's not clear if there's a way of reducing one's current hit points that isn't called damage.

Arguments in favor of it being damage

  • We're told in the definition of "current hit points" that "This number changes frequently as a creature takes damage or receives healing." That can imply that when the number changes downward, it's called "damage".
  • We have this quote from a slightly later section:

    Describing the Effects of Damage

    Dungeon Masters describe hit point loss in different ways.

    Which seems to treat the terms "damage" and "hit point loss" as synonyms.

  • The sentence "Whenever a creature takes damage, that damage is subtracted from its hit points." also could be taken as making the concepts synonyms.

Arguments in favor of just being a change in current hit points

  • Nothing in the rules calls out this reduction in current hit points due to a reduction in maximum hit points as "damage", and generally game terms are used in a somewhat consistent manner.
  • The sentence "Whenever a creature takes damage, that damage is subtracted from its hit points." says that while damage subtracts hit points, it doesn't say that it's the only way to do so.
  • When describing how damage works,

    Damage Types

    Different attacks, damaging spells, and other harmful effects deal different types of damage.

    None of the types listed seem to apply, as we have "rules damage" or "maximum hp reduced damage", and it's clearly a different sort of thing than the types of damage listed.

You're being non-committal. Which is it?

RAW, it doesn't really say. All of 5th Edition has a philosophy that the rules are really just guidelines to help make your own games. I expect DMs to read it however they find reasonable. Per "Rule 0" (Or, if you want an actual citation, the Introduction's "How to Play" section), the DM determines the environment the players are in and results of the players' actions. "The DM decides what happens, often [but not necessarily] relying on the roll of a die to determine the results of an action."

In practice, for a lot of cases, one gets the most sensible answers from the rules if you apply the same rules that apply to damage being dealt (such as falling unconscious when damage takes you to 0 hit points), even though there's a good argument to be made that it isn't technically damage.

For other cases, such as your example of concentrating on a spell, it would be reasonable for a DM to rule that the loss of hit points wasn't something that would affect one's concentration on a spell. Another DM might rule that in fact they would need that Constitution check, because they do in fact suddenly feel "weaker". Both would be completely correct in doing so (though I'd expect a particular DM to generally be consistent in such rulings).

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No, it is different than damage.

As you quoted in the tweet, Damage is a game term involving the loss of hp.

However, Max HP reduction is not damage, it is an effect.

Since you haven't lost HP - there has only been a change in Max HP and still retain all your available hit points. With no damage, there is no concentration break attempt.

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    \$\begingroup\$ @NautArch Damage is not defined as percentage of max HP though. For example, a wild shaped druid, when taking enough damage, can drop back into her full HP for but still have taken damage. Also, if you have temp HP and take damage your current HP doesn't change at all but it still counts as damage right? The key is that he started with 25 HP and ended with 20. \$\endgroup\$ – Rubiksmoose Dec 14 '17 at 19:39
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    \$\begingroup\$ But in your wild shape example, the Druid would have taken damage (while in wild shape) and then reverted, effectively gaining HP. The HP change is the consequence of exiting from wild shape, not from damage. @NautArch is correct: Changing max HP is not the same as losing (or gaining) HP. \$\endgroup\$ – Robbie Dec 14 '17 at 19:46
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    \$\begingroup\$ @NautArch: I knew it was a mistake to reference wild shape. So let us just walk that back. But this is what I'm asking you to answer: does that count as damage. My actual question is not at all about concentration. I've deleted it from the OP. \$\endgroup\$ – Rubiksmoose Dec 14 '17 at 19:51
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    \$\begingroup\$ @Robbie: But when you lose HP you current HP DOES change explicitly as Szega said above. You are correct though that gaining max HP does not increase your current HP though. \$\endgroup\$ – Rubiksmoose Dec 14 '17 at 19:53
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    \$\begingroup\$ @NautArch: You may want to edit out the concentration part of your question since it does not make sense now. But really I am looking for some rules here. I understand your side of the argument. I'm asking you to back it up though. \$\endgroup\$ – Rubiksmoose Dec 14 '17 at 19:57
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Not exactly, it depends on how the maximum hp is reduced.

If HP is reduced because, for example, a spell that was increasing constitution ends, that isn't taking damage and shouldn't interrupt concentration.

However, if an external force reduces Constitution like a tic of poison, that would reduce maximum HP through what the rules call ability damage. Nobody struck you with a weapon, but you suffered "damage" nonetheless and would have to make a concentration check.

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    \$\begingroup\$ Can you support that with some citations? This needs something to make it look more like opinion or personal interpretation. \$\endgroup\$ – keithcurtis Dec 14 '17 at 18:48
  • \$\begingroup\$ Per RAW nothig seems to indicate that this is the case, though having in mind previous editions necromancy it might be RAI. \$\endgroup\$ – LordHieros Dec 14 '17 at 18:51
  • \$\begingroup\$ It is interpretation of course, but yes I can do better at citing. I'll edit this in a moment. \$\endgroup\$ – Trauma Advocate Dec 14 '17 at 18:54
  • \$\begingroup\$ "it depends on how the maximum hp is reduced." Does it though? Why would any one case of HP reduction causing current HP loss be different from another (aside from other effects that have no bearing on my question)? \$\endgroup\$ – Rubiksmoose Dec 14 '17 at 19:00
  • \$\begingroup\$ I made an error, my answer does not apply as well as I thought. In Pathfinder, any concentration checks relevant to decreasing max HP fall under the category of either "injury" or "spell effects". If either of these decreased the max hp, it's a concentration check. I mistakenly thought this was derived from D&D 3.5. Shall I delete this answer? \$\endgroup\$ – Trauma Advocate Dec 14 '17 at 19:13

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