If Curse successfully strikes the opponent, a parasitic bond will be formed between the two Pokémon wherein the cursed Pokémon loses 1/4th of its maximal HP per turn, while the user will be impinged by their own repercussion from it. The parasitic bond is only disformed when the Pokémon switches out or when the battle ends.
The only preclusive counteragent against Curse is the Crafty Shield technique.
The user's repercussions can be dichotomized by whether or not the operative user is a Ghost-type Pokémon; if it is, it becomes more susceptible to repercussions as delineated below.
The user will immediately lose half of its maximal HP; since this value is absolutized to the maximal amount, it would cause the user to faint if they presentially had only half of their maximal HP (rounded-down) or less.
The user's Speed will be downscaled by one stage; however, as a counterbalance, their Attack and Defense stats will also upscale by one stage.