Lipitor is the best-selling drug ever, accounting for $106 billion sales over the last decade, or almost one-quarter of Pfizer’s total. Pfizer has told analysts it is preparing for the loss of Lipitor’s patent with a variety of business moves to preserve market share.

Two generic drug makers are set to compete starting Dec. 1. Watson Pharmaceuticals is making a generic version authorized by Pfizer under a profit-sharing agreement. Pending federal approval, Ranbaxy Laboratories of India also plans to sell a generic version. When a drug’s patent protection expires, the law permits only limited generic competition in the first six months.

After May 31, other generic versions of Lipitor are expected to flood the market, lowering prices sharply. Then, according to the letter from Catalyst Rx, the generic block lifts, Lipitor’s co-payment rises and drugstores are told to fill Lipitor prescriptions with the cheaper generic versions.

Objections to the deal were raised publicly on Thursday in a news release from a group called Pharmacists United for Truth and Transparency, which opposes some tactics of pharmacy benefit managers. The statement called the move “a blatant attempt” by benefit managers to keep Pfizer’s discount while employers still have to pay the full price of the brand-name drug.

David Marley, an independent pharmacist in Winston-Salem, N.C., and spokesman for the group, said the benefit companies and Pfizer would profit at a cost to employers, the government and health care overall.

“That’s why this is unique: We’re talking about blocking a truly less expensive drug, which is why the employers have got to be on top of this,” Mr. Marley said. “This is the most aggressive I’ve seen where they are trusting the employer is not going to figure out the whole game.”