Scientific progress, including in evidence-based medicine, requires all available evidence to be accessed, scrutinised, interpreted and used. Missing or incomplete evidence creates biases and errors in later research. Open science practices are movements and procedures that aim to increase transparency in science production. They aim to make scientific knowledge available, accessible and reusable, benefitting scientific collaboration and all society.1 Open access is a core component of open science that aims to help solve the problem of accessibility.2

Traditional publication behind a paywall can hide evidence from the public, clinicians, policymakers and other researchers. Whether online or print, traditional scientific journals maintain their content behind a paywall, with only abstracts freely available to read.3 Readers access articles by purchasing the individual article, the entire journal issue or through a subscription. These journal subscriptions are purchased by institutions like universities and libraries. However, readers whose institutions cannot afford these subscriptions or who are not affiliated to an institution are often unable to pay to access every article they need. Members of the public and readers in low-resourced countries are disproportionately affected.4

Open access is defined as making a document freely available for anyone to read and, depending on the licence model, share and use (Box 1). Scholarly publishers now offer open access routes for publishing journal articles such as protocols, commentaries, reviews and result articles. The academic community expects these publishers to adhere to the same quality standards as in traditional closed access publication, such as peer review, indexing and permanent archiving. Biomedical research has progressively adopted open access, with yearly increases in the percentage of articles available as open access publications and the number of countries and policies mandating open access.5 6 Online supplemental text 1 summarises national and international open access mandates.

What—Open-access journal publications are scientific articles freely available for anyone to read and, depending on the model, share and use.

Why—Open access makes knowledge available for all, not only those who can afford to pay to read articles or subscribe to journals. Open access is a core component of open science.

When—Some countries now mandate open access for publicly funded health research. However, this is not yet possible in many lower income countries due to open access costs.

Who—Every part of the science ecosystem has a role in making open science publishing possible: governments, funders, research institutions, publishers, educators and researchers.

How—Increasing the proportion of open access articles to allow more people to read involves reducing or eliminating the barriers to the publishing system that authors face, either through funding more general diamond open access venues in a sustainable way, supporting open access fees payments more widely, or both.

Open access has the potential to solve the problem of accessibility. It benefits the final users of the evidence produced, who are patients and healthcare professionals in the case of evidence-based medicine. It also benefits researchers, who can avoid waste from reproducing investigations unnecessarily or having to reinvent procedures that are not clearly explained. To help authors to decide whether to choose open access and how, we summarise the main benefits of open access, the routes for open access and their challenges.