A useful roundup of links and leads for reporters researching the published literature on medical and some environmental health topics is available online at LLRX.com. 
Key among these are the National Library of Medicine databases Toxnet,  Medline, and PubMed.  One of the most productive starting points for combing these and other databases is the NLM Gateway search engine. 
LLRX.com describes itself as "the premier free, independent, one-person produced Web journal dedicated to providing legal, library, IT/IS, marketing and administrative professionals with the most up-to-date information on a wide range of Internet research and technology-related issues, applications, resources and tools." It is produced by Sabrina I. Pacifici, who also publishes a useful blog called beSpacific. 
The September 22, 2008, article  by Gloria Miccioli includes information about the above sources, along with a much wider array of free online sources for researching medical literature.