These are some Frequently Asked Questions about Wikipedia's guidelines on sourcing for medical content, manual of style for medicine-related articles, and how the guidelines and policies apply to biomedical content.
Primary sources aren't completely banned, but they should only be used in rare situations. An individual primary source may be flawed, such as being a clinical trial that uses too few volunteers. There have been cases where primary sources have been outright fraudulent. Furthermore, a single primary source may produce a different result to what multiple other primary sources suggest, even if it is a high-quality clinical trial. Secondary sources serve two purposes: they combine the results of all relevant primary sources and they filter out primary sources that are unreliable. Secondary sources are not infallible, but they have less room for error than a primary source.
This follows a principle that guides the whole of Wikipedia. If a company announces a notable new product, Wikipedia would not cite a press release on the company's website (a primary source) but instead would cite a newspaper article that covers it (a secondary source). The difference with medical information is that the popular press are not suitable sources.
The popular press includes many media outlets which are acceptable sources for factual information about current events, sometimes with significant caveats. It also includes media outlets which are discouraged in all cases because the quality of their journalism is inadequate. However, even high-quality media outlets have disadvantages in the context of medicine.
Firstly, news articles on medicine will frequently be reporting a new medical primary source, such as the results of a new study. This means that they are effectively acting as a primary source, which as explained above makes those articles generally unsuitable for medical information. These articles also tend to omit important information about the study. If a medical primary source is to be cited at all, the academic paper should be cited directly.
Secondly, media coverage of medical topics is often sensationalist. They tend to favor new, dramatic or interesting stories over predictable ones, even though studies that reflect the current scientific consensus tend to be predictable results. They tend to overemphasize the certainty of any result, such as reporting a study result as a conclusive "discovery" before it has been peer-reviewed or tested by other scientists. They may also exaggerate its significance; for instance, presenting a new and experimental treatment as "the cure" for a disease or an every-day substance as "the cause" of a disease. The sensationalism affects both which stories they choose to cover and the content of their coverage.