A Merck & Co. experimental drug has been rejected by the U.S. Food and Drug Administration for the second time in four days.

Shares of Merck fell 5 percent to $39.50 in after-hours trade Monday after closing up nearly 2 percent in the regular trading session.

Merck said Monday evening that the FDA issued a "not approvable" letter, saying it needed more information on Cordaptive, a cholesterol drug Merck had long touted as a key addition to its lucrative cholesterol franchise, which has been under fire this year.

Despite the setback, the Whitehouse Station, N.J.-based pharmaceutical company reaffirmed its 2008 profit forecast of $3.28 to $3.38 per share.

Last Friday, the FDA rejected an allergy drug developed by Merck and Schering-Plough that combines the active ingredients of prescription allergy and asthma pill Singulair — one of Merck's best-selling products — and Schering Plough's Claritin, a former prescription drug now sold over-the-counter. Their joint venture said it is evaluating the agency's response.

The cholesterol drug, also known as MK-0524A, can both lower LDL, or bad cholesterol, and raise HDL, or good cholesterol, according to Merck.

"We plan to meet with the FDA and to submit additional information to enable the agency to further evaluate" the drug's risks and benefits, Peter S. Kim, president of Merck Research Laboratories, said in a statement. "We firmly believe that MK-0524A provides physicians with an important option to manage their patients' cholesterol."

Merck's former cholesterol blockbuster, Zocor, lost billions in annual sales after it faced generic competition in 2006. The company, and partner Schering-Plough Corp. of Kenilworth, N.J., jointly sell two other cholesterol drugs, Zetia and Vytorin — which combines Zetia and Zocor.

The drug makers have been accused of delaying results of a study that showed Vytorin worked no better than much-cheaper Zocor in order to protect sales, which they deny. But amid heavy media coverage and two congressional probes of the issue, Merck has cut its 2008 revenue forecast for the drugs by $700 million.

Cordaptive combines an extended-release form of the B vitamin niacin with a chemical to inhibit a sometimes-intolerable niacin side effect called flushing — redness, burning and tingling of the face.

Niacin has been used to control cholesterol for decades, and an extended-release version called Niaspan, made by Abbott Labs, has been on sale for years.

Dr. John Paolini, head of the Cordaptive development team at Merck, has said the flushing problem has prevented many patients from reaching the most effective dose and caused many others to stop taking niacin or Niaspan altogether. However, Abbott says flushing is generally temporary and can be managed with aspirin.

Some analysts have said that compared with extended-release niacin, Cordaptive had slightly higher levels of substances in the blood that can indicate potential liver damage, muscle damage and blood-sugar problems.

In a 24-week study of about 1,600 patients, reported at a European cardiology conference last autumn, researchers found that compared with dummy pills, Cordaptive produced an 18 percent drop in levels of LDL cholesterol; a 26 percent drop in another type of blood fat called triglycerides, and a 20 percent increase in HDL levels.

Merck also said the FDA rejected Cordaptive as a brand name, so it may use the name Tredaptive in the U.S. The company noted an advisory committee last Thursday recommended that European Union countries approve the drug.

Richard T. Clark, Merck's chief executive officer, said Merck's broad portfolio of medicines and vaccines, "enables us to weather challenges that come our way."