The FDA will issue an advisory today for doctors to use a genetic test to screen patients before prescribing the widely used Ziagen medication, because patients with a particular variation in an immune system gene are at a far higher risk of a severe allergic reaction to the drug, The New York Times reports. The recommendation for the test will be included in a Black Box warning.

A small percentage of Ziagen patients suffer so-called hypersensitivity reactions, either when they start the drug or when they resume using it after some interval, the Times writes, adding that symptoms can include fever, rash, nausea and breathing difficulties. Ziagen, by the way, is also a component of two combination pills, Trizivir and Epzicom.

The association between the variant in the immune system gene and risk of the reactions was first reported several years ago. But the agency says a randomized clinical trial had since definitively proved the value of the genetic prescreening.

The study, published in The New England Journal of Medicine in February, found that prescreening reduced the incidence of suspected hypersensitivity reactions to 3.4 percent, from 7.8 percent. Based on that data it was estimated that 61 percent of people with the genetic variant would suffer a hypersensitivity reaction, in contrast to only 4 percent without the variant, the paper writes. Ziagen was in the news recently over a different health issue - an increased risk of heart attacks.