While the use of preprints (public posting of an early draft of a paper before it’s submitted to a journal for formal review) has long been established in fields like physics and the social sciences, recent uptake in the biomedical world has raised some concerns. When clinical treatment and public health are involved, extra care must be taken to ensure that it is clear to the reader that the work being described has not been peer reviewed. Most preprint servers handle this well, watermarking their preprints and clearly labeling them as preliminary. But little thought seems to have been given to how we cite preprints. Should we treat them the same way that we treat reviewed and published material?

Where you stand on this largely depends on the purpose that you think reference lists in papers are supposed to serve. If you see them as providing empirical support for any statements made in the paper, then the inclusion of preprints in citations likely worries you. An author could make a dubious claim in a preprint that sees no editorial oversight or review, and then cite that claim as an accepted belief in the field in a subsequent published paper. If you see reference lists as a set of links providing further information, then inclusion of non-peer reviewed material isn’t a big deal, caveat lector.
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A journal I work with was recently publicly criticized because they asked an author to remove a preprint from the list of references on an accepted paper. Their reference policy is a traditional one, espoused by many other journals — anything that goes into the reference list must have been peer reviewed. Anything that has not been peer reviewed is treated as a “personal communication” and can be referred to in the paper, but is noted as such. I’ve often heard preprints compared to the equivalent of giving a talk about unpublished work at a meeting, so there is some logic in treating them both the same way when referring to them in a published work.
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