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.