According to the National Library of Medicine,

Preprints are complete and public drafts of scientific documents, not yet certified by peer review. These documents ensure that the findings of the research community are widely disseminated, priorities of discoveries are established and they invite feedback and discussion to help improve the work.

Preprints are posted to "preprint servers." The first and currently largest preprint server is arXiv (pronounced "archive"), developed originally as an FTP server in 1991 as a means to share research in physics.  Preprint servers are now available in almost every field, although biomedicine has been slow to adopt them.

Most preprint servers allow readers to leave comments and also allow authors to update their preprint as needed. Although some preprints go on to be published in journals, many do not.  Posting research to a preprint server provides exposure that this unpublished research would not otherwise receive.  Preprint servers also make research findings freely available, without the paywall of many traditional journals.

However, preprints are not without issues. The major concern in clinical medicine is, because they have not undergone peer review, preprints can spread inaccurate information. This was seen during the COVID-19 pandemic, when unsubstantiated research published in preprints were reported on in the media. To try to combat this, some preprint servers do a preliminary screening.  For example, medrXiv screens submissions for "plagiarism, non-scientific content, inappropriate article types, and material that could potentially endanger the health of individual patients or the public."

Preprint servers that accept papers in the health sciences include, but are not limited to:

• medRxiv (pronounced "med archive") was launched in June 2019 by Cold Spring Harbor Laboratory, Yale University, and BMJ.  It saw explosive growth during the COVID-19 pandemic.
• bioRxiv (pronounced "bio archive") is hosted by Cold Spring Harbor Laboratory and posts preprints in all the life sciences.  It was founded in November 2013. 
• arXiv (pronounced "archive") is the oldest preprint server, launched in 1991 to share physics articles. Hosted by Cornell University, it now includes physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics. It has a small number of articles related to biomedicine. 
• chemRxiv (pronounced "chem archive"), managed by the American Chemical Society, the Chinese Chemical Society, the Chemical Society of Japan, the German Chemical society, and the Royal Society of Chemistry, launched in 2017. It covers all aspects of chemistry, including food chemistry and biological and medicinal chemistry.
• OSF Preprints, from the Center for Open Science, launched in 2016 and covers all research disciplines.  OSF Preprints is segmented by discipline and authors should submit their preprint to the appropriate one, such as Medicine and Health Sciences.
• Research Square is a multidisciplinary preprint platform, accepting research in all scientific areas, including physical, biomedical, and social sciences. It was founded in 2013 and currently owned by Springer Nature. 

SPI-Hub, from the Center for Knowledge Management at Vanderbilt University Medical Center, maintains a list of preprint servers by research area.

Most health sciences journals will accept articles that have been posted to a preprint server, although they require disclosure of this fact. When medRxiv debuted in 2019, numerous journals refused to accept preprints, considering them duplicate publications, but most have changed their policies. The notable exceptions appear to be several orthopedics journals, including Clinical Orthopaedics and Related Research, Bone and Joint Journal, Journal of Bone and Joint Surgery and Journal of Orthopaedic Research, which consider preprints to be duplicate publication--although some of those journals appear to have exceptions to that rule. It is always advisable to check the Instructions to Authors and other policies of journals in your area of research before posting a preprint.

In June 2020, the National Institutes of Health began a pilot project to include preprints resulting from NIH-funded research related to COVID-19 in PubMed Central, with links in PubMed. In January 2023, phase 2 of this pilot was launched and included all preprints, on any subject, that acknowledged NIH support and were posted to medRxiv, bioRxiv, arXiv, or Research Square. If a preprint is subsequently published as a journal article, the records for both versions will be linked if the journal is included in PubMed.

Records in PubMed and Pubmed Central clearly identify an article as a preprint and include a note stating that the article has not undergone peer review. When searching PubMed, it is possible to perform a search that includes only preprints or to perform a search that excludes them by using the preprint filter or publication type [pt] tag.

More information on the NIH Preprint Pilot:

• Pilot Overview
• NIH Preprint Pilot FAQs
• Preprint Tutorial