New warnings have been added to the label of diabetes drug Januvia (sitagliptin) about reports of serious allergic reactions among some type 2 diabetics who have taken it including constricted airways, swelling around the lips and a potentially fatal skin condition called Stevens-Johnson syndrome.

The problems were disclosed by Merck on October 17th, which said they had appeared among patients who have filled more than 2 million prescriptions for Januvia in the year since it was approved for use in the U.S.

Stevens-Johnson syndrome, typically marked by a painful, blistering skin rash that causes the top layer of the skin to die, can be caused by infections, but it also can be triggered by allergic reaction to drugs.

Merck said it had not observed any instances of Stevens-Johnson syndrome in its clinical trials, and wasn't able to estimate the frequency of their occurence among Januvia users or indeed whether the reactions were caused by the diabetes drug.

Merck said some patients who have taken Januvia also have reported suffering anaphylaxis, a severe whole-body allergic reaction that can cause a person to go into shock, and angioedema, a swelling of the skin that leads to welts.

Skin problems are a particular concern because the FDA has held up approval of Novartis Galvus (vildagliptin), like Januvia a DPP-4 inhibitor, in part out of concern about skin lesions that occurred when the drug was tested in monkeys.

But a Merck spokesperson reiterated that no skin problems were observed among monkeys in studies of Januvia, and said there is "no evidence at all" that the allergic reactions in Januvia patients are related to what Novartis and Bristol-Myers Squibb have seen in their monkey tests.

FDA spokeswoman Susan Cruzan, meanwhile, said the agency's the "level of concern is not great" about Merck's addition of the new cautions to its prescribing information. John Buse of the American Diabetes Association termed the additional label information "par for the course."