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I came across the word nocuous. It seems that this word is rarely used (and even the spell-checking of my browser does mark it as a mistake). Noxious, in comparison, is used way more often.

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Interestingly, the opposite statement can be applied to their antonyms innocuous and innoxious. Now the comparison between both shows a clear tendency to the use of innocuous (and my browser's spell-checking marks the latter).

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The question is, in which contexts should I have a preference to nocuous/innocuous and when should I go with noxious/innoxious? Or are possibly only innocuous and noxious in practical use, while the others are rather formal? Perhaps, innocuous is the well-established opposite to noxious in colloquial?

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@BrianHooper Thanks for your edit. Is this really the correct placement, the comma preceding the bracket, although only the part in brackets makes the comma permitted. – Em1 May 25 '12 at 21:27
@Em1, I'd think, to be honest, that no commas are really necessary in this context. Certainly a comma as the first letter in a bracket is very strange. Perhaps that is worth another question? – Brian Hooper May 25 '12 at 21:30
@BrianHooper In addition to the google books data, you can see in the COHA corpus that innoxious has fallen out of use by the 20th century: corpus.byu.edu/coha/x3.asp?xx=1&w11=innoxious&r= – z7sg Ѫ May 25 '12 at 21:43
up vote 4 down vote accepted

Innoxious is used in medical, chemical, and certain manufacturing fields to mean that a substance is not injurious, hurtful, or damaging, especially to tissues. Do a Google search for "innoxious medical" and you get 768,000 results. Of course innoxious is the antonym of noxious and a synonym for innocuous. (Those three words appear in Dorland's Illustrated Medical Dictionary, 30th Ed. Nocuous, however, did not merit an entry.) Both Dorland's and Saunders Comprehensive Veterinary Dictionary include in their definitions of noxious the word "pernicious," which in medical dictionaries is defined as "tending to a fatal tissue." That is, something that is noxious can be very damaging to tissues, irreparably so.

So to answer your question, the most commonly used words (as you suspected) are innocuous and noxious. Innoxious has use in certain fields. Nocuous is just not used.

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Shouldn't tissue be issue? (I don't have medical dictionaries on hand.) – TimLymington May 27 '12 at 15:34
@TimLymington, No. Tissue is correct. It kills the tissue. – JLG Jun 1 '12 at 15:03
Interestingly, in some non-medical dictionaries, ‘innoxious’ is not found at all, while ‘nocuous’ is found, though marked as literary or formal. – Janus Bahs Jacquet Aug 26 '13 at 14:17

An interesting question: I didn't know that there were such words as nocuous and innoxious. I'd suggest the answer would be that very few people have heard of these words, and therefore noxious and innocuous would be preferable.

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I think using either, however in-nocently, would be ob-noxious. – TimLymington May 25 '12 at 22:04

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