Aegerion Pharmaceuticals ($AEGR) has been a rocket of a stock recently, but the share price is based on pure speculation. The stock saw huge gains from positive press releases that were unaccompanied by any promising earnings guidance. The news that the company's new drug would be selling for $295,000 per patient ushered in a huge buying spree, as did news that the drug was approved by various review boards. These well-circulated stories led the stock to fly up to $75 per share from just over $20 earlier this year.
Reality has started to set in though. During the past six months zero news regarding profitability or positive trends in revenues has been released. The positive press, although exciting at a glace, offers few supporting details that the drug will sell so expensively or that the company will be able to actually bring it to market. Without more speculative news to bite on, the inflated share price has begun to fall hard. 
Aegerion dropped every day for the past week, a total sell off of $12 per share or 16%. Today was a brilliant day for the market overall, with the Dow, S&P, and Nasdaq all posting solid positive gains. Not Aegerion though; share prices fell $4.47, or about 7%. The speculation period has ended, and the price will likely fluctuate heavily has the company pushes to bring their pricey drug to market. If you're staying in for the long haul good luck, but the stock will likely remain volatile until drug sales begin, if they ever do.
Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours.