Pedantic time!
Placebo effect applies when the belief provides an effect that is helpful to the body, even if only illusory in nature. It's dangerous because it creates a false sense of healing, which could cause patients to stop taking medication before they've properly healed (or worse, prevented them from looking for medical help at all).
When the effect is harmful instead (like convincing someone that the water they drink is poisoned), it's called the nocebo effect.