Do the words 'insidious' and 'pernicious' have the same meaning? And can they be used in place of each other?

On some places on the web (eg : http://www.dailywritingtips.com/insidious-vs-invidious/), it says that the two words have different uses, but the oxford dictionary defines them both similarly.

Oxford dictionary links : insidious and pernicious

  • Dictionaries have limited space to explain all the nuances. – Mitch Sep 14 '16 at 17:27

I'll go again with Merriam-Webster 3rd International:


  1. watching for an opportunity to ensnare: insidious tempter
  2. intended to entrap or trick: insidious plot
  3. acting by imperceptible degrees: insidious disease
  4. having gradual, cumulative, and often hidden effects; insidious pressures of modern life
  5. subtle: insidious charm

For synonyms it gives 'sly', but obviously that won't work everywhere, mostly 1, 2, possibly 5.

Pernicious has none of these meanings. It basically means destructive or harmful. in the names of diseases (pernicious malaria, pernicious anemia) it means especially severe forms of the disease.

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Insidious suggests a lying in wait or a gradualness of effect or approach and applies especially to devious and carefully masked underhandedness.

  • a part of an insidious conspiracy to undermine the world- Edmund Wilson
  • an insidious tempter

Pernicious is more often applied to things that harm exceedingly or irrepairably by evil or by insidious corrupting.

  • a pernicious influence
  • pernicious propaganda

(excerpts from MW dictionary of synonyms)

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  • Can something be called insidiously pernicious ? :) – pranavk May 22 '17 at 6:56

The actual OED does not make them synonyms, defining "pernicious" as extremely harmful (from the Latin per (intensive prefix) + nex, necis death) and "insidious" as treacherous (from the Latin insidere, to lie in wait). Ambushers lying in wait for you can't intend anything but harm, but they needn't mean to kill you. And something can be pernicious and readily apparent. Consider the sentence from The Province of Jurisprudence by John Austin:

What appears pernicious to one person may appear beneficial to another.

That doesn't mean that the two can't go together. Pernicious anemia was named before its cause was understood, at a time when it was invariably fatal. But it is also an insidious disease, developing slowly with multiple symptoms that evade an easy diagnosis.

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Good research. Your first reference does a good job of pointing out the subtle distinctions. In particular, it points out that "insidious" is

  • "(... neutral, but the word is rarely used except in a negative sense.)"

The Oxford dictionary does define them similarly, but not identically:

for "insidious", notice that it mentions both the gradual nature and the harmful effect together.

For "pernicious", it mentions first the harmful effect and then says "especially" about the gradualness.

So according to Oxford Dictionaries:

  • "insidious" things always come on gradually and always have harmful effects.

  • "pernicious" things are always harmful**, and often (but not necessarily) come on gradually.

So you could use either word for things that are very hamful and come on gradually or subtly.

However, for something that is only, say, annoying (like that song you just can't get out of your head) you could say it is "insidious", but not "pernicious". It insinuates itself, not innocently, but innocuously.

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  • Thanks for your response! So, if something is harmful and shows its effect with a slow, subtle build-up, can I use any of the two words to describe it? Or, is one better than the other? – Ojas Jul 16 '15 at 6:33
  • It's your choice. See how you like the way it sounds to say it. For instance, if you're writing a poem, it might depend on whether you need it to rhyme with "hideous" or with "vicious." But that's writing style, not grammar or usage. – Brian Hitchcock Jul 16 '15 at 7:12

Insidious an insidious change or problem spreads gradually without being noticed, and causes serious harm

an insidious trend towards censorship of the press

Pernicious very harmful or evil, often in a way that you do not notice easily

the pernicious effects of poverty.

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Pernicious is just a more severe and harmful form of insidious. Something that is insidious in nature may or may not be pernicious. The bermuda grass may be taking over my yard insidiously, but i would not say it is pernicious. I would save pernicious for some diseases that come on gradually but have severe consequences.

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  • 2
    Welcome to EL&U and thank you for your answer! On this site we strive to produce the best answers to questions and it is important that they are backed up by relevant information. Can you find some links to a dictionary, for example? – BladorthinTheGrey Sep 14 '16 at 16:18
  • Pernicious is bad. Insidious is sneaky bad. The answer misses this distinction. – MetaEd Sep 14 '16 at 17:09

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