TRENTON, N.J. — Merck & Co.’s longtime chief financial officer plans to retire in July now that the company has regained its footing, the drugmaker said Tuesday.

Judy C. Lewent, 58, has been CFO for 17 years and started at Merck in 1980.

She was a key contributor to the drugmaker’s corporate restructuring unveiled in late 2005. The plan called for focusing research on nine disease areas and speeding up development of promising drug candidates to restore Merck’s financial health, along with across-the-board job cuts and other money-saving strategies.

Whitehouse Station-based Merck, the maker of osteoporosis treatment Fosamax and Singulair for asthma and allergies, said it will consider internal and external candidates as it seeks a replacement.

Lewent’s retirement comes as Merck struggles with falling revenues, mushrooming litigation over its withdrawn painkiller Vioxx, and as other blockbuster drugs face generic competition. While its stock price has recovered since pulling Vioxx from the market in September 2004 due to increased cardiovascular risks, restructuring costs and legal fees continue to pull down profits.

Merck said Lewent had been mulling retirement for several years.

“However, the company was undergoing a particularly challenging period and she did not feel that she could leave until it was back on firmer ground,” Merck spokesman Ray Kerins said. With the restructuring off to a promising start, he said Lewent decided to retire.

Lewent joined Merck in 1980 and rose quickly, becoming vice president and treasurer in 1987. Three years later, she moved up to chief financial officer and vice president for finance.

Among her contributions was helping steer Merck away from focusing almost solely on internal research to instead obtaining more potential drug compounds through licensing deals and acquisitions. She led talks resulting in several important alliances and acquisitions of potential drugs, and also chaired the governing boards of several of Merck’s joint ventures, including ones with Schering-Plough Corp. and AstraZeneca PLC.

“Judy is a world-class CFO,” Richard T. Clark, Merck’s chief executive officer, said in a statement. “Her business and financial acumen, keen strategic thinking and analytical skills have made her an invaluable advisor and colleague.”

Merck shares were up 24 cents to $44.51 in afternoon trading on the New York Stock Exchange.