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 time for a while. 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 expression, surprised expression 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.
List of poisoning attacks
By enemies and bosses