FOR some years the big drugmakers have been dreading an approaching "patent cliff"—a slump in sales as the patents on their most popular pills expire or are struck down by legal challenges, with few new potential blockbusters to take their place.

This week the patent on the best-selling drug in history expired—Lipitor, an anti-cholesterol pill which earned Pfizer nearly $11 billion in revenues last year.

In all, blockbusters with a combined $170 billion in annual sales will go off-patent by 2015.

What is supposed to happen now is that lots of copycat firms rush in with "generic" (ie, chemically identical) versions of Lipitor at perhaps one-fifth of its price.

Patients and health-care payers should reap the benefit.

Pfizer's revenues should suffer. The same story will be repeated many times, as other best-selling drugs march over the patent cliff (see chart).

But generics makers may face delays getting their cheaper versions to market.

Ranbaxy, a Japanese-owned drugmaker, struggled to get regulators' approval for its generic version of Lipitor, and only won it on the day the patent expired.

More important, research-based drug firms are using a variety of tactics to make the patent cliff slope more gently.

Jon Leibowitz, chairman of America's Federal Trade Commission (FTC), is concerned by drugmakers filing frivolous additional patents on their products to put off the day when their protection expires.

Another tactic is "pay-for-delay", in which a drugmaker facing a legal challenge to its patent pays its would-be competitor to put off introducing its cheaper copy.

In the year to October the FTC identified what it believes to be 28 such settlements.

American and European regulators are looking into these deals.

However, legal challenges against them have faltered, and a bill to ban them is stuck in Congress.

According to BernsteinResearch, under the deal between the two drugmakers Pfizer will receive about 70% of Watson's revenues from its approved copy of Lipitor.

More unusual, Pfizer has cut the price of its original version, and will keep marketing it vigorously.

So Ranbaxy faces not one, but two competitors.

This strategy has precedent, says David Risinger of Morgan Stanley, but the scale and structure of Pfizer's scheme is unmatched.  

Patients with a special discount card from Pfizer will make co-payments (their contribution to the pills' costs under their health plan) of just $4 for a month's worth of the original Lipitor, compared with about $10 for many generic medicines.