From Mushak & Elliott (2015):

Pharmaceutical companies hire “medical education and communication companies” (MECCs) to create sets of journal articles (and even new journals) designed to place their drugs in a favorable light and to assist in their marketing efforts (Sismondo 2007, 2009; Elliott 2010). These articles are frequently submitted to journals under the names of prominent academic researchers, but the articles are actually written by employees of the MECCs (Sismondo 2007, 2009). While it is obviously difficult to determine what proportion of the medical literature is produced in this fashion, one study used information uncovered in litigation to determine that more than half of the articles published on the antidepressant Zoloft between 1998 and 2000 were ghostwritten (Healy and Cattell 2003). These articles were published in more prestigious journals than the non-ghostwritten articles and were cited five times more often. Significantly, they also painted a rosier picture of Zoloft than the others.