Lipitor, the world's best-selling drug, goes off-patent in the USA today, and observers are watching keenly to see how damaging the loss will be to Pfizer and how much generics firms will benefit.

Pfizer is fighting hard to keep hold of a decent portion of the huge revenues generated by Lipitor (atorvastatin) - about $5 billion in the USA alone last year. The drugs giant has struck deals with a number of pharmacy benefit managers and health plans in the USA to offer the drug at heavily-discounted prices in a bid to beat off competition from generic versions of the cholesterol pill.

During a briefing which was widely reported by news services, David Simmons, Pfizer's president of emerging markets and established products, said that the firm will compete with generics over the next six months by offering the blockbuster to patients for just $4 for a one-month supply. He claimed that "more than a third of the current patients who are on Lipitor would like to stay on Lipitor”, adding that in the current system, when I go into pharmacy and if I am going to be dispensed a generic, I have no idea where it’s coming from".

Watson's authorised generic shipped

This aggressive stance means that Ranbaxy Laboratories and Watson Pharmaceuticals, which have 180 days to sell their versions before the market gets flooded with cheap Lipitor, need to get their skates on. The latter has moved fast and confirmed it has started shipping an authorised generic as part of an exclusive agreement with Pfizer, which will receive a share of sales.

Watson noted that the agreement runs until November 30, 2016 , adding that for the twelve months ending September 30, Lipitor had sales of approximately $7.8 billion, according to IMS Health data.

The Indian drugmaker's position is more complicated. Ranbaxy has been in dispute with the US Food and Drug Administration since 2008 when the agency hit the firm with an import ban over manufacturing violations at two plants in India. Several reports are claiming that the company is prepared to pay a fine in the region of $350-$400 million to resolve the problem and lead to a final approval from the FDA for its Lipitor copy.