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 Medivation (NASDAQ:MDVN), a biopharmaceutical company focused on developing drugs to treat serious diseases, including cancer, dipped as much as 16% after reporting its fourth-quarter earnings results after the bell last night and issuing fiscal 2014 sales guidance for its lead drug, Xtandi.

So what: For the quarter, Medivation reported $96.6 million collaborative revenue with its partner Astellas Pharma for Xtandi. This includes sales of Xtandi within and outside the U.S., as well as milestone payments received due to predetermined sales markers for advanced prostate cancer treatment Xtandi. Overall, this represented a 160% increase in year-over-year revenue. Medivation was also able to report net income of $2.8 million, or $0.03 per share, compared to a year-ago loss of $31.7 million, or $0.43 per share, in the fourth quarter. Both results absolutely decimated Wall Street's expectation of $72.7 million in revenue and a $0.05 EPS loss. However, the wheels fell off the bus when Medivation issued its full-year sales forecast for Xtandi of $500 million-$535 million, which many investors and analysts found conservative. On the heels of that guidance, Jefferies downgraded Medivation to hold from buy.

Now what: It's not uncommon for biopharma stocks that could be sitting on a blockbuster stock to sandbag the guidance a bit. It's also not a huge surprise to see Medivation shares under pressure considering how much they've run higher over the past year. One thing investors will want to consider, though, is just how impressive its PREVAIL results were in chemotherapy-naive patients last month. The results demonstrated that Xtandi practically ran circles around the placebo in terms of reduced risk of death and radiographic progression and nearly tripled the length of time before patients began chemotherapy. If Xtandi gains this additional pre-chemo indication, which seems likely, it is well on its way to becoming a blockbuster, in my opinion, an excessive weakness from downgrades could be the perfect opportunity needed to buy into this growth if shares continue to fall.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.