Advisory panel meetings at the Food and Drug Administration are typically pretty dramatic. The FDA usually calls them when it needs a little help deciding whether to approve a drug or not: Sitting on the fence always creates drama. The agency even puts out previews of the featured event in the form of documents it sends to the panel members, which gives investors a first look at what the FDA might be thinking.

For Johnson & Johnson's (NYSE:JNJ) and Bayer's Xarelto, the documents were a mixed bag -- a preview of a comedic tragedy perhaps. The drug works well in preventing blood clots, but it has a higher likelihood of causing excessive bleeding. There's also some worry about the potential for liver damage.

Balancing a drug's better performance with the increased risk of side effects seems to be a common theme lately. Eli Lilly's (NYSE:LLY) prasugrel has had the same problems, and it's still waiting for an approval.

sanofi-aventis (NYSE:SNY) is sitting on the sidelines, cheering for a delay or even an outright refusal to approve the drug, because it makes the blockbuster blood thinner Lovenox. And look who's sitting up in the nosebleed seats, also rooting for a delay: Generic-drug makers Momenta Pharmaceuticals (NASDAQ:MNTA), Novartis (NYSE:NVS), and Teva Pharmaceutical (NASDAQ:TEVA). The companies -- Momenta and Novartis are a team -- have been trying to bring generic versions of Lovenox to market for years. Any drug that cuts into Lovenox's sales threatens potential sales of their generics.

The real drama will come Thursday, when the FDA advisory panel meets. If the panel shrugs off the safety data, Xarelto likely will be home free. Otherwise, the drug may have a hard time getting approval.

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