The US Food and Drug Administration has announced a labelling update for the widely prescribed blood thinner warfarin, advising doctors that patients' genetic makeup can affect how they respond to the drug.

Manufacturers of warfarin, the generic version of Bristol-Myers Squibb’s Coumadin, are to add similar information to their products' labelling, the FDA said, noting that the labelling change “highlights the opportunity for healthcare providers to use genetic tests to improve their initial estimate of what is a reasonable warfarin dose for individual patients”.

The agency estimates that two million people start taking warfarin in the USA every year to prevent blood clots, heart attacks and stroke. However it is a difficult drug to use because the optimal dose varies and depends on many risk factors including a patient's diet, age and the use of other medications.

Patients who take a dose larger than they can tolerate are at risk of life-threatening bleeding, noted the FDA, while those who receive too low a dose are at risk of equally dangerous clots. Dosing is particularly important at the beginning of therapy, when problems in adjusting the dose can lead to complications such as bleeding. Warfarin is the second most common treatment – after insulin – used to deal with adverse drug events.

Health care professionals who prescribe warfarin regularly check to see if the drug is working properly by ordering a test called the PT or prothrombin time that evaluates the blood's ability to clot properly. The results are measured in seconds, the FDA said and its commissioner Andrew von Eschenbach, said that the approved labelling change “is one step in our commitment to personalised medicine. By using modern science to get the right drug in the right dose for the right patient, FDA will further enhance the safety and effectiveness of the medicines Americans depend on".