Widely used blood thinner warfarin will come with new instructions explaining that people with certain genes may need a lower dose to take the drug safely, U.S. health officials said on Thursday.

The change is part of a move toward personalized medicine in which doctors tailor treatment based on genetic makeup, the Food and Drug Administration said.

Warfarin is sold by several generic makers and by Bristol-Myers Squibb under the brand name Coumadin. An estimated 2 million U.S. patients start taking the drug each year to prevent blood clots that may cause heart attacks and strokes.

Some patients, however, experience dangerous bleeding. Experts say about two in 100 patients taking warfarin will have a serious hemorrhage. Excessive bleeding from the drug is one of the most common prescription-related causes of emergency room visits, studies have shown.

Current data suggests that people with variations in two specific genes - CYP2C9 and VKORC1 - face a higher chance of bleeding. People with those variations can be identified with a blood test available at major laboratories across the country that costs between $125 and $500, FDA officials said.

Information about the genes is being added to warfarin's prescribing instructions and may help physicians decide whether to adjust the dose lower for certain patients, FDA officials said.

Doctors will not be required to test for the genes.

Although genetic testing can currently identify who has these genetic variants, more studies are needed to explore the precise starting dose for these patients, said Larry Lesko, director of the FDA's clinical pharmacology office.

Other factors such as age, weight and diet also may influence bleeding risk.

Bristol-Myers agreed to the FDA's request to add the genetic testing information to Coumadin's label, spokesman Jeff Macdonald said.

The company is committed to working with regulatory authorities as science evolves to appropriately inform physicians on the use of our medicines, he said.