Merck & Co , which plans to buy rival drugmaker Schering-Plough Corp later this year, reported lower-than-expected sales and earnings that it attributed to the global economic slowdown.

Many of the company's biggest products suffered sales declines, including asthma treatment Singulair and Gardasil, a vaccine against cervical cancer whose revenue declined by a third.

Merck, whose shares fell almost 3 percent in premarket trading, said on Tuesday it earned $1.46 billion, or 67 cents per share. That compared with $3.33 billion, or $1.52 per share in the year-earlier first quarter when it received a big payment from marketing partner AstraZeneca .

Excluding special items, Merck earned 74 cents per share. Analysts on average expected 78 cents per share, according to Reuters Estimates.

Global sales fell 8 percent to $5.39 billion, well below the Reuters Estimates forecast of $5.76 billion.

Sanford Bernstein analyst Tim Anderson said nearly half of Merck's revenue miss was due to weak vaccine sales. He called the quarterly results disappointing.

Merck reaffirmed it expects per-share earnings this year of $3.15 to $3.30, reflecting a decline of between 3.5 percent and 7.8 percent from last year. But it trimmed its 2009 sales forecast to between $23.2 billion and $23.7 billion. Previously, Merck had forecast $23.7 billion to $24.2 billion.

Our first-quarter results in part reflect the impact of the difficult global economy on patients, providers and payors, but we remain on track to meet our full-year earnings guidance, Chief Executive Richard Clark said in a release.

Clark said the company's planned acquisition of Schering-Plough would help assure sustainable growth in coming years.

Global sales of Merck's Fosamax osteoporosis drug, now facing generic competition in the United States, plunged 44 percent to $261 million. Sales of Gardasil, facing competition overseas from GlaxoSmithKline's Cervarix vaccine, shrank to $262 million.

Combined sales of cholesterol fighters Zetia and Vytorin, which Merck sells in partnership with Schering-Plough, fell 23 percent to $945 million, following several studies that questioned the drugs' safety and effectiveness.

Singulair, Merck's biggest product and formerly one of its fastest-growing drugs, slipped 4 percent to $1.1 billion, hurt by concerns the pill may increase risk of suicide.

Combined revenue from blood pressure drugs Cozaar and Hyzaar fell 1 percent to $839 million.

Sales of Januvia, a relatively new diabetes drug, surged 51 percent to $411 million. But rival products that work through the same mechanism of action are in advanced stages of development.

Merck on Tuesday also announced it no longer plans to seek U.S. approval this year for its experimental treatment for migraine headaches, called telcagepant. The drugmaker did not provide a reason, but said it would provide an updated timeline later.

Schering-Plough earlier on Tuesday reported higher-than- expected first-quarter earnings as it took a hatchet to its research and administrative spending.

Merck shares were trading at $24.51 in premarket activity, from their closing price of $25.22 on Monday. Schering-Plough shares edged slightly lower.

(Reporting by Ransdell Pierson; Editing by Derek Caney and Maureen Bavdek)