New data show sales of cholesterol blockbuster Lipitor plunged by half barely a week after the world’s top-selling drug got its first U.S. generic competition. That’s despite a very aggressive effort by Lipitor maker Pfizer Inc. to keep patients on its pill, which once generated sales of $13 billion a year. Pfizer has been contracting with insurance plans to subsidize both patient copayments and costs for insurers, to keep their costs at or below what generic Lipitor would cost. Miller Tabak analyst Les Funtleyder says the drop in the number of Lipitor prescriptions filled is not as great as he expected. Lipitor lost U.S. patent protection on Nov. 30, and two generic versions costing about one-third less are now available.