Baxter International Inc., the major producer of heparin in the U.S., has stopped making the blood drug, due to several reports of allergic reactions coming from patients.

The U.S. Food and Drug Administration has received about 350 complaints from patients using injectable heparin, since the end of 2007. Patients reported side effects such as difficult breathing, nausea, and rapid drop of blood pressure, which can be fatal. Four of the patients have died.

Heparin is a blood-thinning drug, derived from pig intestines. It is mostly used before surgeries, especially heart surgery, and it is also given to dialysis patients before treatment. The drug has helped millions of people around the globe, who suffered from blood-clotting problems.

But unfortunately, the problems reported to the FDA have appeared in the cases when large doses of heparin were injected intravenously. It seems these situations involved only heparin produced by Baxter International Inc., so the company has now stopped the production of multiple-dose vials.

As heparin made by Baxter represents half of the entire U.S. supply, meaning 35 million vials annually, doctors worry about the shortage of the drug on the market.

"A recall would result in an immediate and severe shortage," said Dr. Jenkins, director of the FDA's office of new drugs, for The Wall Street Journal.

"It potentially could become a critical issue as far as dialysis patients," said James L. Bailey, a nephrologist at Emory University in Atlanta.

During the dialysis process, the patient’s blood is run through some tubes while it gets cleansed and heparin is necessary to prevent the blood from clotting.

FDA recommends the doctors in U.S. hospitals to use the drug in very small amounts and over a long period of time, as apparently, the allergic reactions have occurred when heparin was used in large doses.