Shares of drug maker Pfizer Inc. (NYSE: PFE) have been tumbling in early trading after reporting this morning a plunge of 18% in its first-quarter profit. The company's earnings numbers have been dragged down by lower sales of blood-pressure drug Norvasc and the allergy drug Zyrtec.

The company said its quarterly profit dropped to $2.78 billion, or 41 cents per share on strong generic competition. These numbers are down from $3.39 billion, or 48 cents per share reported in the same period a year ago. Excluding items, Pfizer's earnings would have come in at 61 cents per share, missing analysts' predictions for a profit of 66 cents per share in the quarter.

Pfizer's quarterly revenue also slipped 5% to $11.85 billion.The company attributed the revenue decline to its loss of U.S. exclusivity for blood pressure drug Norvasc. However, the drop in revenue could have been even worse if the drug maker hadn't benefited from the weak dollar, Pfizer stated. Analysts expected the company to show sales of $12.06 billion in the first quarter, according to Reuters Estimates.



Sales of Lipitor, the world's best selling drug, lost 7% to $3.1 billion as doctors have been switching patients from Lipitor to a rival drug sold by Merck & Co. (NYSE: MRK).

Despite its weak earnings figures, Pfizer reaffirmed its guidance of earnings per share, excluding certain items, of $2.35 to $2.45 this year. But this was not enough to impress investors who pushed the stock down over 3% in early trading.

Eliza Popescu is a financial writer for the online investment advisory service Investor's Observer.