Damage Reduction (DR) is a measure of how much weapon damage a character or monster can ignore before losing hit points. The DR value is subtracted from each incoming hit, to a minimum of zero damage received. If you have a DR 10 and get hit for 9 points of damage, you take no damage at all. If you have a DR 10 and get hit for 15 points of damage, you only take 5 points of damage. DR applies only to physical weapon damage, although there is a similar concept of Resist Energy that applies to energy/elemental damage from spells, enchanted weapons elemental damage, and non-physical traps.
Passive DR may be bypassed by the properties of a particular attack. Some forms of DR are vulnerable to materials (like Adamantine or Silver), alignments (like Chaos or Good), damage type (like Slashing), or just "Magic".
 Player DR
 Passive DR
Passive DR is visible on your character sheet, and also sometimes on buffs in your buff bar. Multiple DR entries may be listed on your sheet; only the highest one which is not bypassed by a particular attack's properties (like Adamantine) applies. The actual DR used against an attack is reported in the combat log as final/net damage dealt followed by damage reduced.
For DR that can be bypassed, the relevant property is listed after the number on the character sheet, in the form of "DR 6/Adamantine" or "DR 5/Evil". If there is no property which can bypass the DR, a "-" is listed instead.
- Example 1 - A character with an equipped Invulnerability item (DR 5/Magic) also has the temporary Ironskin Chant song (DR 6/-) from a Bard.
- They are hit by a Mephit, creatures which bypass DR Magic. However, since they have DR6/- from Ironskin Chant, the attack damage will still be reduced by 6.
- Example 2 - The same character is buffed with the Stoneskin spell (DR 10/Adamantine).
- The higher DR 10/Adamantine will apply to incoming damage instead of the Invulnerability or Ironskin Chant until the Stoneskin is depleted.
 Passive DR Sources
Passive DR can be provided from many sources, including:
- Warforged Adamantine Body, Improved DR Feats, or Warforged Improved DR enhancements (DR/Adamantine)
- Barbarians get +1 DR/- at levels 2, 5, 8, 11, 14, 17, 20 and a +1 Enhancement available at levels 3 and 7
- Clerics or Favored Souls can get 5/- DR by level 3 using the Warpriest Enhancement called Resilience of Battle.
- Favored Souls are granted DR10/X at level 20, where X is a type of damage based on their Faith enhancements that are chosen at level 1
- Monks gain DR10/Epic at level 20 via the granted feat Perfect Self
- Adamantine Armor DR/- (1 from shields or light armor, 2 from medium armor, and 3 from heavy armor)
- Ironskin Chant (DR6/-) song from Warchanter Bard
- Stacking Half-Elf Improved Damage Reduction with the Barbarian Dilettante Feat (DR 1/- or 2/-).
- The Stoneskin spell (DR 10/Adamantine), Angelskin spell (DR5/Evil)
- Magical items of Invulnerability (DR 5/Magic), or of Axeblock, Hammerblock, and Spearblock
- Certain named items, like the Bloodrage Symbiont (DR5/-) or Golden Greaves (DR3/-)
These passive DR sources do not stack with each other; only the highest DR not overcome by an attack is taken into account.
The Warforged Feats and Enhancements that improve DR are special cases. These benefits explicitly say they grant DR/Adamantine or improve your existing DR/Adamantine. They stack with your single strongest other source of Adamantine DR, whether it comes from a Warforged racial benefit or something else.
 Monsters Bypassing DR
Monsters generally deal one physical type (bludgeon, pierce, slash) of damage with an attack, which will bypass the corresponding physical DR type.
There are only a few ways a monster can bypass other types of passive player DR, loosely following the 3.5 edition DR rules:
- Monsters with DR Magic will bypass DR Magic with their melee (not ranged) attacks (generally magical monsters such as mephits and gargoyles).
- Beginning around CR12, most monsters are considered to have Magical weapons and will bypass DR Magic. This renders DR Magic almost useless beginning around Sands/Gianthold.
- Epic raid bosses will bypass DR Epic with their attacks.
- Monsters with alignment DR (Good, Evil, Lawful, Chaotic) will bypass the alignment DR of the opposite type. Example: Most Devils have DR Good, and so will bypass DR Evil.
There are currently no known monsters that can bypass material type DR (Adamantine, Byeshk, Cold Iron, Crystal, Mithral, Silver).
Player characters (but not monsters) gain additional "Active DR" when they actively block by holding down the shift key. Active DR is displayed as "Blocking DR" in your inventory sheet. Active DR cannot be bypassed except by melee attacks from behind (benefits of blocking apply only to melee attacks against the character's front 180°).
Active DR stacks with any Passive DR (from the single, highest applicable source).
 DR and PRR
Damage Reduction is applied before the reduction from your Physical Resistance Rating.
 Monster DR
DDO includes monsters with Damage Reduction. Monster DR may require specific material, damage types, and/or alignment to bypass the DR and deal full damage. Additionally, some monsters are vulnerable to magic weapons only (DR/Magic). Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes DR/Magic.
Damage types are bludgeoning, piercing and slashing.
Alignment types are Chaos (or Anarchic), Evil, Good and Law (or Axiomatic).
- An Aligned weapon mimics all alignments.
- A Green Steel Weapon mimics Evil alignment for the purpose of bypassing DR even if not actually Evil.
Material types are Adamantine, Byeshk, Cold Iron, Crystal, Mithral and (alchemical) Silver.
- A Metalline weapon is enhanced to mimic all material types except Crystal.
Weapons with both Silver (from Silver or Metalline) and Good (from Pure Good/Holy/Flametouched Iron/Righteous) attributes are popularly known as "Harry Beaters," based on the nickname of Arraetrikos, the end boss of The Shroud raid. Such weapons can bypass this boss' DR of Silver+Good.
Some examples include:
- Bearded Devils and Orthons (devils) require Good Aligned or Silver weapons
- Clay Golems require Bludgeoning and Adamantine weapons
- Crimson Foot Spiders require Piercing weapons
- Eladrins require Cold Iron or Evil Aligned weapons
- Flesh Renders (demons) require Good Aligned and Cold Iron weapons
- Thaarak Hounds require Lawfully Aligned weapons
- Maruts require Chaotically Aligned weapons
- Ghostly Skeletons require Good Aligned weapons
- Iron Golems and Flesh Golems require Adamantine weapons
- Rakshasas require Good Aligned and Piercing weapons
- Skeletons require Bludgeoning weapons
- Vampires require Silver weapons
- Zombies require Slashing weapons
 Damage Reduction Scaling
Damage Reduction Scaling works similarly to Dungeon Scaling, except it applies to damage reduction, and applies to both players and monsters. Note unlike Dungeon Scaling this only happens in Casual, Normal and Hard Dungeons. Not elite/raid/epic (Dungeon scaling does apply on elite).
 Player versus monster
This scaling helps players who otherwise can't bypass or overcome a monsters damage reduction at all, still deal some damage. For any physical attack that would otherwise be completely negated, this scaling allows them to deal 50%/25% of the damage they would have otherwise dealt if the enemy had no damage reduction.
- Normal: DR scaling fully applies - you deal 50% of the damage if your attack would have otherwise been negated.
- Hard: DR scaling applies, but at half strength - you only deal 25% of the damage.
- Elite/Epic/Any Raid: DR Scaling is disabled. Attacks may be reduced to zero.
Examples (Normal difficulty):
- Player swings his +4 Greatsword at a Ghostly Skeleton which has DR100/Good. He would have dealt 40 damage, with all 40 points being stopped by the DR. However due to DR scaling, he instead deals 20 damage to the skeleton.
- Player swings his +4 Greatsword at a Ghostly Skeleton which has DR100/Good. This time he would have dealt 135 damage. Since this is above the damage reduction of the enemy, scaling does not need to happen, thus he deals 35 damage.
 Monsters versus player
If a player has some form of damage reduction a monster can't bypass, it's only applied without scaling being taking into account, if that amount would result in less damage then just the scaling. This means that damage reduction can seem far less effective when solo in scalable dungeons, as enemy damage is heavily scaled down, but this only happens after damage reduction.
- Monsters attack is calculated out to deal 20 damage, if no dungeon scaling was in effect at all. Player has DR10/Silver and the monster doesn't bypass this. The player is in the dungeon solo, as such the monster only deals 20% of his regular damage, thus 4 damage. The calculation is applied like this:
- A) 20 damage dealt dungeon scaling scales that damage down to 4 points.
- B) Damage reduction is calculated ignoring scaling. 20 damage, 10 was stopped. 10 damage is dealt.
- C) The lowest of the 2 damage values is applied. This is displayed in a masked way to fool the player: Enemy hit you for 4 damage, 10 was stopped.
Essentially damage reduction does nothing for the player in this scenario due to how it is coded.