GlaxoSmithKline's cervical cancer vaccine Cervarix is facing a delay in winning approval in the United States, putting it further behind Merck's rival product Gardasil.
Glaxo, Europe's biggest drugmaker, said it had received a "complete response letter" from the US Food and Drug Administration (FDA), which is issued when the regulator has completed the review of a medicine, but still has questions.
"GSK will work closely with the FDA to prepare its response, which it intends to submit to the agency as soon as possible," Glaxo said in a statement.
Cervarix is one of Glaxo's biggest new drug hopes and is already approved in 45 countries, including the 27 European Union member states. But it is trailing rival vaccine Gardasil, which is already well established in the United States.
Analysts had been expecting the FDA to issue a decision on Cervarix by next month. In cases such as this, the regulator usually takes up to six months to respond, once it has received the answers to its questions.
