Damage and healing effects are reversed. Taking damage restores HP, while healing reduces it.
Reverse (逆転, Gyakuten?) is a status effect that causes attacking and healing to have the opposite effect for afflicted target; when an enemy is in Reverse, all attacks directed at it will heal rather than injure. Reverse bears some similarities to the Zombie status ailment present in many Final Fantasy games in that it causes healing magic to act as offensive magic.
In the 3D version, Dr. Lugae is the exclusive user of Reversal Gas, which inflicts Reverse on all party members and enemies. He'll start by using Restore to heal the party, making it hard to defeat Dr. Lugae.
In the Zodiac versions, Reverse is listed as a Green Magick 2 License, costs 50 LP and 20 MP, and is bought at the Clan Provisioner for 7,000 gil. Reverse can be learned by the four mage job classes—White Mage, Black Mage, Time Battlemage, and Red Battlemage.
One accessory, the Nihopalaoa, uses an effect similar to Reverse, causing all items used by the holder to do the opposite of their normal effects (e.g. items that normally heal HP deal damage instead, items that normally cure status effects will inflict those status effects instead).
Reverse affects all regular attacks, damaging and healing spells and most Esper attacks. Reverse does not affect Drain and Leech, Syphon, Poison, Sap, Regen, Phoenix Down, Knot of Rust, Dark Matter, Quickenings or Technicks. The following Esper skills are also exceptions: Zalera's Kill, Shemhazai's Soul Purge and Zodiark's Final Eclipse.
Although the status is classed as "negative", it can be beneficial in long battles by casting Reverse on one or all members of the party so all attacks from the enemy will heal them. Due to this advantage, the spell is considered "cheap" by some players. The only drawbacks to the status are that if any party members have an Esuna Gambit active, they will immediately attempt to remove the status due to its negative classification.
Also, if the player has a gambit set to heal using Cura or Curaja, in which the character would normally be healed, they would instead be hurt. If a character has Reverse status and they are stopped, Reverse will not wear off until Stop does. This can be used for strategic advantage against enemies that use the gambit Foe:nearest visible, by placing a stopped and reversed character near the foe, letting the character attract all of the enemy's attacks, while the other party members can attack from distance with ranged weapons.
Zalera's spell, Lv.5 Reverse can inflict reverse to a character whose level is divisible by 5.
Kuribu, the final boss in the Lost City of Amdapor (Hard) will cast Reverse on itself after it is revived once, causing its healing spells to target the party and cause damage instead. Its Regen spell will inflict the Bleed status on anyone who is within the area of effect, Cure IV will cause massive damage the closer the party is to Kuribu, and Cure III will cause moderate damage on two random players plus others if they are within the area of effect. When Kuribu is revived a second time, it will alternate between using its Reverse strategy and using normal healing on itself.