Big pharmaceutical companies once tended to disregard rare diseases, preferring to develop blockbusters for common ailments like diabetes and high cholesterol. But with sales growth slowing and older blockbusters losing patent protection, the companies have become more willing, even eager, to sell specialized drugs.

“This is the first step in the pursuit of a formal strategy around orphan drugs and rare diseases,” David Simmons, president of Pfizer’s established products business unit, said in an interview.

While there might be few patients with each disease, he said, “collectively, it’s a very large patient population with a great unmet medical need.”

Besides, Genzyme has proved that even a drug for a rare disease can generate big sales if the price of the drug is high enough. Only about 5,700 people in the world are being treated with Cerezyme. But since the drug costs about $200,000 a patient each year, sales exceeded $1 billion last year.

Pfizer is not alone in pursuing treatments for rare diseases. In October, GlaxoSmithKline announced a deal with Prosensa, a Dutch company, to develop drugs for Duchenne muscular dystrophy. And in June, Novartis won approval from the Food and Drug Administration to sell its drug Ilaris as a treatment for cryopyrin-associated periodic syndrome. Only about 300 Americans suffer from that syndrome, an inflammatory condition caused by a gene mutation.

Philip Nadeau, an analyst at Cowen & Company, called the entrance of Pfizer as a competitor a “modest negative” for Genzyme. However, Mr. Nadeau said, “even Pfizer’s marketing prowess is unlikely to relieve” safety concerns some doctors have about the Protalix drug. The main concern is that because it is produced in plant cells, taliglucerase may spur immune reactions in some patients.

Genzyme’s stock rose a bit less than 1 percent, to $51.11. The company announced Monday that it had resumed shipping some newly manufactured Cerezyme. Manufacturing had been shut down in June after a virus contaminated the company’s factory in Boston.