There's a medicine cabinet full of drug makers reporting earnings this week, and Pfizer's (NYSE:PFE) one of the latest in the bunch. While it had a dose of what some may consider bad news -- its first-quarter net income was half of last year's figure, while revenues just barely missed analysts' expectations -- that doesn't change the fact that delivered a pretty impressive revenue jump, with the help of several acquisitions.

Namely, Pfizer's sales increased 47% to $12.49 billion. It reported earnings of $2.33 billion, or $0.30 per share, as opposed to $4.67 billion, or $0.76 per share, in the same time frame last year. (Last year, however, some onetime gains beefed up the quarterly numbers.)

However, if you back out costs related to the acquisitions, including Pharmacia and Esperion, the drug maker reported net income of $3.98 billion, or $0.52 per share, exceeding the consensus view.

How did Pfizer's stable of well-known drugs fare? Some of them, of course, face intense competition. Much anticipated has been the heated rivalry facing erectile dysfunction drug Viagra. After all, within recent months, Viagra has faced GlaxoSmithKline (NYSE:GSK) and Bayer AG's (NYSE:BAY) Levitra, as well as Eli Lilly (NYSE:LLY) and ICOS' (NASDAQ:ICOS) Cialis.

With Viagra no longer enjoying the pleasure of a monopoly, sales have gone a bit flaccid, having dropped 12%. Though it seems the defection rate could be worse, Pfizer sees the threat of a fickle customer who will follow the newest pretty face and is taking new marketing measures. According to The Wall Street Journal, loyal customers of the erectile dysfunction drug can expect that for every six Viagra prescriptions filled, they'll get the seventh free. It's an odd turn to incentive-based marketing that seems alien to pharmaceuticals and underlines the recreational aspect of the drug.

When it comes to other familiar Pfizer names, the company reported a 15% sales slip of antibiotic Zithromax; the well-known antidepressant Zoloft increased its sales, but mostly internationally.

Meanwhile, Pfizer's star, cholesterol drug Lipitor -- one of that class of popular drugs called statins, and renowned for being the best-selling drug in the world -- enjoyed a 19% boost in sales. It now boasts a 43.4% market share. Sales of blood pressure drug Norvasc increased 16%.

Today's numbers imply the wisdom of the mergers and the continued success of many of its important drugs. However, despite the good news, Pfizer didn't budge its full-year guidance. It still sees earnings of $2.13 per share for the year. Investors weren't impressed, and Pfizer shares sagged in recent trading. While Pfizer still has a whole portfolio of strong, well-branded meds, maybe some little signs of weakness crept in.

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Alyce Lomax does not own shares of any of the companies mentioned.