The cholesterol-fighting Lipitor lost its patent protection Wednesday, which means cheaper generic versions of the top seller will soon be available. Two versions of the medication are expected to be on the market in the next six months.

In most cases, the new generics gobble up the drug’s market share while the drugmaker quietly shifts its focus to another product.

But Lipitor’s maker, Pfizer, is hoping to hold onto 1 million of the estimated 3 million Americans taking the drug.

“More than one-third of patients currently taking Lipitor would like to stay on Lipitor,” said David Simmons of Pfizer, citing internal company research.

Lipitor (chemical name atorvastatin) brings in about $11 billion per year for Pfizer, one-sixth of the company’s revenue.

In an effort to keep its customers, Pfizer is offering patients a discount card to get Lipitor for $4 a month, via their website, LipitorForYou.com. The pharmaceutical giant is also keeping its supply levels steady up until the last minute, negotiating deals in an effort to block pharmacists from dispensing the generic version, and paying pharmacies to counsel patients that Lipitor lowers bad cholesterol more than rival drugs.

An estimated 21 million prescriptions for Lipitor were dispensed in the first half of 2011.