Joining Merck in 1980, Lewent developed a new planning model in 1983 based on the Monte Carlo method, as more traditional financial valuation methods seemed not to take account of the long-term nature of pharmaceutical risks. In 1990, on her appointment as CFO, Lewent took on responsibilities for Merck's overall financial and strategic development, and for licensing matters.
Since then Lewent has also been responsible for Merck's collaboration with Johnson & Johnson in a consumer pharmaceuticals joint venture, for the setting up of the animal health company Merial (a joint venture with the French pharmaceutical company Sanofi-Aventis), and for Merck's ongoing relationships with AstraZeneca and DuPont. She chaired the governing boards of several of these joint ventures.
Between 26 July and 07 Aug 2007, Lewent sold 200,000 shares of Merck stock for about $10,262,000, according to Securities and Exchange Commission filings.
In February 2007, Merck has announced she will retire and leave the company in July.[4]