pernicious

/pərˈnɪʃəs/
adjective
  1. Having a very harmful or destructive effect, often in a gradual or subtle way.
    • The pernicious effects of gossip can destroy friendships over time.
    • Social media can have a pernicious influence on young people's self-esteem.
    • The pernicious weed spread through the garden, choking out all the flowers.
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