Derek Lowe mentions the amazing trail of names that have ended up at GlaxoSmithKline, and has a suggestion in Naming of Names:
Someone should do a family-tree chart of all the mergers and buyouts over the years in the drug industry. Judging from this example, though, I don't think that someone is going to be me.
After the merger between Pharmacia & Upjohn and Monsanto (Searle) created Pharmacia, they produced a lovely family tree that showed the history of the companies that were now part of Pharmacia. It showed fairly untrameled histories of Searle (until purchased by Monsanto in 1985) and Upjohn (until merged with Pharmacia in 1994). The European company that was Pharmacia before 1994 was essentially a holding company of Swedish, German and Italian companies that they gobbled up over most of the 20th century. It's good to know that this isn't a new phenomenon. Now these are part of Pfizer, which includes Warner, Lambert, Parke and Davis. I am sure there are many others, since Pfizer got its start in fine chemicals, rather than pharmaceuticals specifically.
For those that want to build such a tree, most pharma firms include a timeline or history on their "About" pages, but most of these focus on their products, rather than their mergers and acquisitions.