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 VIVUS (NASDAQ:VVUS) have soared Monday by as much as 12% after the company scored a distribution deal with Express Scripts (NASDAQ:ESRX).

So what: Express Scripts has joined the Qsymia Certified Home Delivery Pharmacy Network, broadening VIVUS' distribution for its weight-loss pill that was launched just last month. Qsymia will be available through mail order. With Express Scripts being the country's largest pharmacy benefit manager, this deal is a boon for VIVUS.

Now what: Qsymia is the first to enter the obesity drug market in 13 years, and investors are clearly excited about its potential to boost the top line. The deal today is a good sign that VIVUS has a leg up on Arena Pharmaceuticals (NASDAQ:ARNA) and its competing Belviq drug for the time being. Belviq has already been cleared by the Food and Drug Administration, but hasn't been released into the market pending DEA clearance. For now, VIVUS has a first-mover advantage.

VIVUS may have just made a huge move, but is Arena that far behind in the obesity drug battle? The Motley Fool's most knowledgeable biotech writer has compiled details on Arena Pharmaceuticals into one clear report, which details the key developments you must be watching if you want to profit on its stock. There's a huge opportunity in Arena, but you have to be careful. To get instant access to the Fool's expert analysis today, simply click here now