When you get a drug from your physician, how do you know it'll work for your specific problem? How do you know it's even safe? Well, your doctor tells you it should work and is safe, but how do they know? They look at the published data on the drug, which in turn is based on experimental medical studies of that drug, studies that are published in peer-reviewed medical journals.
Now, some studies are good, some are bad. It's really quite difficult to make a good experimental study; there's many, many pitfalls along the way. A bad study may have too few patients to give any good data, or have some methodological problem, or are mishandling the statistics, or are even not really asking the right questions at all. A good study avoids all that. In medicine you often have the question of impartiality. When a study and the people working on it are independent of the company making the drug all is well and good. But when the study is paid for by the pharmaceutical company, or the main researchers are getting a lot of money or junkets, then impartiality can and should be questioned. There's many (far too many) examples of studies that have been altered to suit the funding company, had data faked to conceal some problem, or the whole study silently dropped altogether when the results turn out to be unfavorable.
Medical journals in particular really want to be vigilant to this kind of thing. Decent journals require upfront declaration of all connections (monetary or otherwise) between the researchers and the company and editors try to determine if there's a conflict of interest that may invalidate the study. Reviewers try as best they can to figure out if the study is any good - if the statistics hold up, if there's any problem with the methodology and so on. It's common even for good, solid papers to have to be rewritten with further clarification of some point, or require additional data and analysis to cover some problem area. Bad, biased papers can (and do) slink through this net it's fairly unusual.
Merck figured out they could bypass this pesky quality control by paying Elsevier, a major scientific publisher with a rather shady record for integrity, to create a fake journal. Which they happily did, even going so far as to recruit an actual physician to be on the "review board". Apparently not a single paper was ever actually peer-reviewed; they were lousy studies that would have been rejected by a real journal, or thinly disguised marketing material - fake papers, in other words, the kind of thing researchers lose their jobs and careers over. Merck then used these fakes to sell their products to physicians. Since the fake journal was published by Elsevier nobody would question the source; no reputable publishing house would actively deceive its readership - it's customers and contributors - after all? Of course that's exactly what Elsevier has done.
So here we have a problem: this only came to light by chance. There's no way of knowing how much seemingly solid data about Merck products safety and efficiency is actually fraudulent. So we - I and you and everyone else - have no way of knowing which of their product claims are real and which are made up by the company and then deceptively pushed as honest research. Here is a partial list of Merck products; do you know which are backed as safe or effective by faked research reports? Neither do I. You want to use any of their medicines? Neither do I.
As for Elsevier - they're a large company, one of the biggest scientific publishers in the world. This is a list of journals. 2325 journals, according to that link. How many other of those journals are paid-for fakes masquerading as real research journals? No idea. And if they accept payment for setting up fake journals, do they accept payment for placing bad papers in otherwise good journals too? Should it surprise us if they did? The Elsevier ethical guidelines are here; this is especially rich:
Elsevier takes its duties of guardianship over the scholarly record very seriously. Our journal programs record "the minutes of science" and we recognize our responsibilities as the keeper of those "minutes" in all our policies, including the guidelines we have adopted to support editors, reviewers and authors in performing their ethical duties.
We are committed to ensuring that advertising, reprint or other commercial revenue has no impact or influence on editorial decisions.
I won't be publishing in Elsevier journals in the future, and I will no longer accept reviewer invitations to one of their publications. My considered opinion, as a researcher, is that Elsevier can go fuck themselves.