deleterious
Meaning: harmful
Usage: She experienced the deleterious effects of running a marathon without stretching her muscles enough beforehand.
noxious
Meaning: harmful, unwholesome
Usage: Environmentalists showed that the noxious weeds were destroying the insects’ natural habitats.
pernicious
Meaning: extremely destructive or harmful
Usage: The new government feared that the Communist sympathizers would have a pernicious influence on the nation’s stability.