There are several ways one can attempt to access clinical trial data for a published study, depending on the type of study and what entity holds the data, but no certainty that one will be able to access all or part of the data. There are four broad source categories: investigators, sponsors, regulators and to some extent, public repositories. If a trial is published, asking for data from the authors of the trial publication is an easy first step (but be prepared that this may not result in data).  Some sponsors maintain their own register (e.g. GSK and Merck), and some have created consortia to create a single entry portal for industry-sponsored trials. The latter require protocols and requestors go through a screening process before being granted access to data. All have limitations on data release. Regulators, chiefly EMA, require requests for documents of compounds regulated prior to January 1, 2015 but EMA is prospectively making more recent data available through its Clinical Data Publication website. FDA and Health Canada are slowly falling into line but FDA medical, statistical, and other reviews that discuss trials submitted to them, are easily accessible from Drugs@FDA.

Public repositories such as Zenodo and Dryad are freely accessible, but searching them for clinical trial data is not easy.  The Drug Industry Documents archive (formerly known as DIDA) also holds a selection of clinical study reports, formerly confidential internal memoranda, and other trial data.