The patent rights on the world's best-selling drug of all time, cholesterol-reducing Lipitor, have expired, opening the way for the sale of cheaper copies of the medication.

U.S.-based drugmaker Pfizer has had exclusive rights to sell Lipitor since 1997, and since then has sold about $100 billion worth of prescriptions for the white, oval-shaped pills. But on Wednesday, Pfizer's patent expired, giving other pharmaceutical companies a chance to produce identical versions of the drug and attempt to sell it at a lower price than Pfizer.

One company, American-based Watson Pharmaceuticals, immediately launched its generic version of the drug under an arrangement in which Pfizer will still manufacture the drug, but Watson will sell it and the two companies will share the profits for the next five years. An Indian pharmaceutical, Ranbaxy, is also planning to produce a copy of Lipitor in the next six months.

Pfizer, meanwhile, says it intends to continue to sell the drug. It has offered its current customers sharp price discounts to persuade them to continue to use the branded Lipitor, rather than a copied version of it, in order to hold on to some of its $10 billion in annual sales. Pfizer has given some Lipitor patients discount cards that will cut their costs for it to $4 a month, down from the $25 they often paid until now.