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Merriam-Webster implies both are correct:

side effect (without hyphen)

side-effect (with hyphen)

Which is more common? My go-to litmus test, google searching both and comparing the number of results, does not work here. My specific context here is the body of a medical research paper.

To avoid debates and polling: I'm not asking which is correct (it seems both are acceptable), I'd like to know which is more common.

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  • 4
    Ngram works for this. – Brian Donovan Dec 21 '16 at 15:10
  • I think that hyphenating "side-effect" only makes sense if you're treating them as a compound noun, such as "We are lowering dosages to reduce the side-effect frequency." – rajah9 Dec 21 '16 at 15:40
  • @BrianDonovan: Hmm, maybe it does and maybe it doesn't. There are possible sentences that contain "side effect" which do not use it the same way "side-effect" is used. The number may be negligible, or it may not, but your assertion that "Ngram works for this" remains, to my mind, unproven. – Robusto Dec 21 '16 at 15:55
  • @rajah9: I think you're trying to shoe-horn in a completely irrelevant principle. There's actually no significant usage preference one way or the other with such noun adjunct usages, but Brian's chart looks just the same if you change it to plural (so it can't include noun adjunct usages). – FumbleFingers Dec 21 '16 at 15:56
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A great resource open to you for looking at the frequency of words or phrases is Google Ngram Viewer.
As Brian Donovan said in the comments, this Ngrams analysis answers the question quite well: Ngrams Screen Capture

As this shows, side effect written as two words has been a lot more common throughout the past half-century with the hyphenated version used much less. That said, the recorded uses of side-effect are not negligible, some people do hyphenate.

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