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.
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).
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?