pernicious
adj.adj. causing great harm or damage. You use this to describe things that are very bad for you, like a bad habit or a dangerous idea.
adj. causing or likely to cause great harm or damage. Often used to describe ideas, habits, or substances that have a slow but destructive effect.
The pernicious effects of smoking are well known.
The spread of the pernicious rumor caused a lot of unnecessary panic among the local residents.
The philosopher argued that the pernicious influence of wealth on the mind was the primary reason for the decline of the republic.
From Middle English, from Old French pernicios, from Latin perniciōsus (“destructive”), from perniciēs (“destruction”), from per (“through”) + nex (“slaughter, death”).
Analyzable as pernīx ("swift") + -ious.