Poison

Poison is a status that gradually drains the player's health. Blue and Orange Snakes can poison the player, as can the poison bogs in Sky Heights. When a player is poisoned, they lose 1/2 heart of health about every 5 seconds. Their hearts turn purple, and purple spirals fade in and out near their face. Poison is especially dangerous in Bubble Cave, where Blue Snakes occupy some of the narrow, winding paths, and the enclosed maze-like structure may make leaving the area harder. It can also inconvenience players struggling to find the route through the deeper poison bog in Sky Heights. Bringing some antidotes to both areas can help.
Players can cure poison by using an antidote or leaving an area–if the player is far from the area exit and doesn't have any antidotes, periodically pausing to use healing items can be helpful. Antidotes will not work if the player is currently standing in a poison bog.
Compare Temperature, another status that can gradually drain health.