This is another blow for the drugmaker, which is still reeling from the controversy over its Avandia diabetes pill and the subsequent lost revenue. Now, the FDA has issued a so-called ‘complete response letter’ for its Cervarix vaccine. Although it’s not clear, though, whether the agency wants additional trials, leaving open the possibility that the unexpected delay in approval can last anywhere from just six months to up to two years.

“We have already started addressing the questions and will be engaged in discussions with the FDA to finalize our responses,” Barbara Howe, vp and director for Glaxo’s North American vaccine development, says in a statement. “Our discussions with the agency continue to be positive and constructive, and we are working diligently to resolve any outstanding questions to bring CERVARIX®to the U.S.market.”

Cervarix is one of Glaxo’s biggest new meds and is already approved in 45 countries, including the European Union, but the US market remains its most important battleground, where the drugmaker already trails Merck and its Gardasil vaccine for the human papillomavirus that leads to cervical cancer. Gardasil targets four strains of HPV - including two causing cancer and two causing genital warts, while Cervarix targets only the two cancer strains.

“What we don’t want is the FDA saying we want to see new data because that would add in one to two years potentially,” Navid Malik, an analyst at Collins Stewart in London, tells Bloomberg News. “It would be disappointing if the delay was substantial, but it could be something like a six-month delay if the FDA only wants relatively minor information.”