There are a few attacks in Super Smash Bros. Ultimate that have a poison effect. The poison effect makes opponents hit by the effect slowly take damage over a small period of time. Although this makes it similar to the Flower effect, the poison effect can't be removed via performing an action, making it more consistent, though it generally deals less damage at a slower rate to compensate. Any fighter under the effects of poison will have a purple tint and purple bubbles rising from them while they take damage, and they might display an angry, surprised, or sad expression.
In addition to all the attacks that normally inflict poison on opponents, there is an Adventure Skill in World of Light that gives all smash attacks a chance of having this effect. Certain Spirit Battles will have a stage with a poison floor or poison fog. The damage can be reduced or negated by the spirit skills Poison Damage Reduced and Poison Immunity respectively, used as healing with Poison Heals, and used to increase a character's damage and speed with Poison Power Up. The poison fog can be removed with the Fog Immunity skill.
List of poisoning attacks
By enemies and bosses
By stage hazards
There are no stages that feature poison, but the Spirit Battle conditions "the floor is poisonous" and "the stage is covered in a poisonous cloud" will inflict poison while in contact with the poison. Poison floors deal 4% every 0.33 seconds (20 frames) and poison clouds deal 3% every 1 second.