This is a case of getting the proverbial good with the bad. The FDA approved Merck’s hot, new Januvia diabetes pill for use with another class of drugs in order to control blood sugar. At the same time, the drugmaker disclosed receiving reports that some patients developed serious complications, including a potentially fatal skin condition called Stevens-Johnson Syndrome, and serious allergic reactions.

Until today, Januvia was largely regarded as an unqualified success for Merck, which has largely convinced Wall Street that a strong rebound is under way, thanks to the diabetes pill, new vaccines and other meds being developed. Januvia, in particular, has been hailed as on track to become a blockbuster, racking up $144 million in sales in the second quarter, even though some docs may be waiting for long-term data to better gauge safety.

In a statement, Merck says it is diffcult to “reliably estimate” the rate at which side effects may be occuring or a link to Januvia. John Amatruda, Merck’s vp for clinical development, tells Reuters that no cases of Stevens-Johnson Syndrome were seen in any of Januvia’s clinical trials; only a “a few” cases have been reported since Januvia was approved in October 2006. “It’s not possible to establish a causal relationship,” he says. “We don’t know what other medications these patients were on.”

The drugmaker noted reports of serious allergic reactions, such as anaphylaxis and angioedema, had already been included in Januvia’s package insert label, which will be updated to caution patients to stop taking Januvia if they develop an allergic reaction. An FDA spokeswoman tells Reuters that the labeling changes were warranted, but that the level of concern wasn’t high. Interestingly, the FDA delayed approval of Novartis’ Galvus which, like Januvia, is a DPP-4 inhibitor, after skin lesions had been identified in monkeys during a preclinical animal study.

Originally, Januvia was approved for use on its own or with metformin or another class of diabetes drugs called thiazolidinediones, such as Takeda Pharmaceutical’s Actos and Glaxo’s Avandia.