Open-Access Mega-Journals (OAMJs) are a relatively new and increasingly important publishing phenomenon. The journal Medicine is in the unique position of having transitioned in 2014 from being a 'traditional' highly-selective journal to the OAMJ model. This study compares the bibliometric profile of the journal Medicine before and after its transition to the OAMJ model. Three standard modes of bibliometric analysis are employed, based on data from Web of Science: journal output volume, author characteristics, and citation analysis. The journal's article output is seen to have grown hugely since its conversion to an OAMJ, a rise driven in large part by authors from China. Articles published since 2015 have fewer citations, and are cited by lower impact journals than articles published before the OAMJ transition. The adoption of the OAMJ model has completely changed the bibliometric profile of the journal, raising questions about the impact of OAMJ peer-review practices. In many respects, the post-2014 version of Medicine is best viewed as a new journal rather than a continuation of the original title.