Some 8.7 million Americans take Lipitor for high cholesterol. Starting Wednesday, they might get a different-looking pill when they go to fill a prescription and pay much less for it.

That's the day the U.S. patent on Pfizer Inc.'s blockbuster drug expires, allowing generic versions of the statin medication to go on sale. Many patients have been eagerly awaiting the lower insurance copayments that come with a generic medicine. Lipitor, the biggest-selling prescription drug of all time, generated sales of $10.7 billion last year.

The switch is generating confusion among some patients.

Richard Allen, a retired real-estate developer from Bainbridge Island, Wash., says that his 77-year-old wife has been taking Lipitor for years with positive results and worries that the generic won't work as well as the branded version. He says he has called his Medicare plan, local pharmacy and even Pfizer to find out whether his wife could stay on Lipitor and what the cost would be. "They didn't know anything," Mr. Allen says.

Adding to the confusion, Pfizer has embarked on an ambitious plan to keep the brand-name drug's sales alive after the advent of generic versions, striking bargains with pharmacy-benefit managers to keep dispensing Lipitor for a time at generic prices and starting up a direct-mail service.