With Lipitor already over the patent cliff, Pfizer ($PFE) has been working hard to hang onto exclusivity for its famed erectile dysfunction drug, Viagra. In its latest bid to block generic competition, Pfizer has sued Indian drugmaker Torrent Pharmaceuticals for patent infringement.

Torrent is seeking FDA approval for generic Viagra (sildenafil) before the U.S. patent on the product expires in October 2019, according to Pfizer's complaint, which was filed in Manhattan and obtained by Bloomberg. Pfizer seeks to permanently bar Torrent from making or marketing the drug.

Pfizer has good reason to protect Viagra. The drug brought in $1.88 billion in sales last year. Even though that marked an 8% drop year over year, the product still accounted for 3.6% of the company's total revenues. Viagra's patent has already lapsed in Europe, Korea, and other regions, prompting dozens of generic drugmakers to flood the market with copycats.

Even as it fights to extend Viagra's life, Pfizer is showing a willingness to compromise with generic rivals. After a long patent battle with Teva Pharmaceutical Industries ($TEVA), Pfizer recently agreed to let the Israeli generics giant launch its version of Viagra two years early--in December 2017. Teva will pay a royalty to Pfizer in return.

But Pfizer has also made it known that it won't tolerate copycats that overstep their bounds. Last year it filed a trademark infringement suit against Hanmi Pharmaceutical in Korea, alleging that Hanmi's pill is an exact copy of Viagra's blue-diamond shape. And Pfizer has fought back against counterfeiters by launching its own website to sell the real Viagra directly to patients.

Still, the generic assault against Pfizer continues, and not just with Viagra. Last week, a U.S. court invalidated a key patent on Celebrex, the company's $2.9-billion-a-year pain drug, opening the door for generic competition 18 months earlier than expected. Generics makers Mylan ($MYL) and Actavis ($ACT) are gearing up to launch their versions of Celebrex (celecoxib) as Pfizer prepares to appeal.