An acquisition, a drug approval, and the release of year-end financial results constitute a busy week for any company. After completing all these undertakings, management at drug developer Shire Pharmaceuticals (NASDAQ:SHPGY) must have been pretty pleased this weekend.

For 2006, revenue was up 12% to $1.8 billion, led by $860 million in sales of its flagship attention-deficit hyperactivity disorder (ADHD) drug Adderall XR. Gross margins were flat at 86%, though that's still quite impressive. Operating cash flow gained 38%, and adjusted earnings hit $1.70 per American depository share.

Shire hasn't been afraid to make relatively large acquisitions in the past, and the big news this week was its decision to acquire partner New River Pharmaceuticals (NASDAQ:NRPH) rather than share revenue on sales of ADHD treatment Vyvanse. Vyvanse was approved for marketing by the FDA on Friday, and it will be launched in the second quarter. Shire agreed to pay $2.6 billion in cash, or $64 a share, for New River, representing a 10% premium over New River's already lofty valuation.

Even before the New River acquisition, Shire has positioned itself as the most dominant force in the ADHD market, with a wide range of products. 2007 may see Shire launch as many as three ADHD drugs in the U.S. alone, if it can overcome FDA regulatory hurdles. How much these new products eat into its existing ADHD drug sales, versus stealing competitors' share of the market, is something investors should watch closely. Shire is only forecasting 4%-6% prescription growth in the indication.

Despite the cloudy financial outlook based on all these moving parts, Shire is nonetheless guiding for 2007 revenue growth of 20%. I wouldn't put too much stock into this estimate, though, since the market's acceptance of Vyvanse is still unknown.

With generic drugmaker Barr Pharmaceuticals (NYSE:BRL) now able to introduce a generic version of Adderall XR in 2009 -- a drug accounting that accounts for nearly half of Shire's revenues -- I'm sure Shire will work hard to switch patients over to Vyvanse and its other new ADHD treatments like Daytrana. How good of a job Shire does in switching patients over to these new treatments will define its future, considering that most of its pipeline candidates are in early stages or for niche diseases.

Barr is a Motley Fool Stock Advisor pick.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article.