Federal regulators have delayed a decision on whether to approve Merck & Co.’s experimental vaccine to prevent shingles and reduce pain from the common nerve disorder, sending Merck shares down Friday.

Zostavax is one of four vaccines that company executives have been touting for the last couple years. If successful, it could help replace revenues that will be lost when the struggling drugmaker’s top seller, cholesterol treatment Zocor, faces generic competition this spring.

The U.S. Food and Drug Administration was to have decided whether to approve sales of Zostavax by the end of this week, 10 months after Whitehouse Station, N.J.-based Merck submitted its application, said Merck spokeswoman Christine Fanelle. This week, FDA notified the company its new target date for a decision is May 25.

Merck shares fell 31 cents, to close at $35.10 Friday on the New York Stock Exchange.