Preprints vs Peer-Reviewed Papers

Two stages of the same paper

Many health stories now start with a preprint. The National Library of Medicine (NLM) describes preprints as complete and public drafts of scientific documents that are not yet certified by peer review. [1] A peer-reviewed paper has been judged by independent experts, chosen by a journal, before publication. This page explains the difference and what to do with each. It is education, not medical advice.

What a preprint is for

NLM says preprints let findings reach the research community widely, establish priority of discovery, and invite feedback to improve the work. [1] Many preprints are later submitted to journals, so later versions may appear after review. [1] That is normal, and it also means the early version can differ from the final one.

NLM states plainly that readers of preprints should be aware that any aspect of the research, including the results and conclusions, may change as a result of peer review. [1]

How much changes?

A study of preprints from bioRxiv and medRxiv that were later published in journals compared the two versions. The number of figure panels and tables changed little. The conclusions in the abstract changed in a discrete way for 7.2% of non-COVID-19 preprints and 17.2% of COVID-19 preprints, and most of those changes did not qualitatively alter the paper's conclusions. [2]

So the typical preprint survives review reasonably well, but a meaningful minority of conclusions do change, and nobody can tell in advance which. The study looked only at preprints that were eventually published; preprints that never appear in a journal are not covered by those numbers.

What peer review does not guarantee

Peer review adds scrutiny, but it is not proof. A paper can pass review and still be contradicted later. Among 49 highly cited clinical studies published in leading journals between 1990 and 2003, 16% were contradicted by later studies and another 16% had found stronger effects than later ones. [3] Small trials were more likely to be among them. [3]

Peer review also does not check that a trial matches its registered plan. The COMPare project found extensive outcome-reporting problems in five leading journals that all endorse reporting guidelines. [4] See registered trials vs published results.

How to treat each in practice

A preprint reported by a news outlet as settled fact deserves extra caution. A press release or a conference abstract is a different kind of document again; see press release vs conference abstract vs peer-reviewed paper.

A quick routine

1. Check whether the paper is labeled a preprint and whether a journal version exists.
2. Look at who was studied (cells, animals or people) and how many; see reading early-stage research.
3. Look for a registry entry and whether the outcomes match.
4. Wait for repeat studies or systematic reviews before treating a result as settled.

If a finding seems relevant to your health, talk with a clinician rather than acting on a draft.