harmful or destructive
pernicious - harmful
Typically, something ‘pernicious’ is harmful but in a way you do not notice easily. Its subtly damaging effects may not be immediately apparent but instead appear slowly over time. For that reason, the word implies irreparable harm caused by the gradual corruption or undermining of something.
I regularly warn my students of the pernicious effects of sitting indoors and playing computer games.
You may not fully appreciate the perniciousness of poverty until you are older and more vulnerable.
Continued cuts in government funding will inevitably have a pernicious impact on our health service.