The US Food and Drug Administration said the manufacturers of drugs containing the active ingredient Carbamazepine have agreed to add a warning to the drugs' labelling when prescribed to Asian patients.

The warning to be added to the label recommends that, before starting therapy with the drugs, patients with Asian ancestry get a genetic blood test that can identify a significantly increased risk of developing a rare, but serious, skin reaction.

Carbamazepine is a drug used for treatment of epilepsy, bipolar disorder, and neuropathic pain. It is sold under the brand names Carbatrol, which is manufactured by Shire PLC, Equetro by Validus Pharmaceuticals Inc. and Tegretol manufactured by Novartis AG.

"Science is now letting us individually treat patients based on how their body might react to a drug," said Janet Woodcock, M.D., FDA's deputy commissioner for scientific and medical programs, chief medical officer, and acting director of the Center for Drug Evaluation and Research. "When being considered for treatment with carbamazepine, genetically high-risk patients can be given a test that will help their health care providers make personalized drug treatment decisions and help avoid potentially serious skin reactions."

The prescribing information for these drugs already includes a warning that for all patients starting carbamazepine therapy, regardless of ethnicity, rare but severe and sometimes life-threatening skin reactions can occur. These life-threatening skin reactions include toxic epidermal necrolysis and Stevens-Johnson syndrome, characterised by multiple skin lesions, blisters, fever, itching and other symptoms.

The risk of these reactions is estimated to be about 1 to 6 per 10,000 new users of the drug in countries with mainly white populations. However, the risk is estimated to be about 10 times higher in some Asian countries.

The skin reaction warnings will be moved to the current boxed warning section of the labelling. The new recommendation that health care providers give patients with Asian ancestry a genetic test before starting treatment will also be added to the boxed warning section.

To screen for this genetic marker, a patient's blood can be drawn by a health care provider and the test administered at a laboratory. It is estimated that about 5 percent of patients being considered for treatment with carbamazepine are of Asian ancestry and would need to have this test.

Studies have found a strong association between certain serious skin reactions and an inherited variant of a gene, HLA-B 1502, an immune system gene, found almost exclusively in people with Asian ancestry. Patients testing positive for this gene should not be treated with carbamazepine unless the benefit clearly outweighs the increased risk of these serious skin reactions.

Patients who have taken carbamazepine for more than a few months and not experienced any skin reactions are unlikely to ever experience these reactions, regardless of ancestry or genetic test results. Patients currently taking carbamazepine who are concerned about these skin reactions should not stop taking the drug without first consulting their health care provider.