Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of drug developer Amarin (Nasdaq: AMRN) were slumping today, losing as much as 12% in intraday trading after the company had its first drug approved by the U.S. Food and Drug Administration (yes, you read that right).

So what: An FDA approval for a small developer like Amarin would normally be cause for investors to party like it's 1999. So why in the world is Amarin's stock falling today? It would appear that investors may be disappointed at the restrictions that the FDA put on Amarin's drug, Vascepa. The restrictions will limit the overall addressable market for the drug out of the gate, which is an outcome that would understandably damper investor enthusiasm.

Now what: It's not like investors who have been riding Amarin recently haven't been rewarded -- the stock has roughly doubled so far this year (even after today's drop). The task now is for investors to go back to their spreadsheets and figure out what the longer-term financial implications are for the narrower market for Vascepa and determine whether the dip in the stock today provides is a good time to load up or join the rush to the exits.

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