CHICAGO (Reuters) - Walgreen Co (WAG.N: Quote, Profile, Research) on Tuesday posted an 8.3 percent increase in February sales at stores open at least a year, as an extra day in the month due to leap year and strong Valentine's Day sales overshadowed a decline in flu-related prescriptions.

Excluding the extra day, same-store sales rose 4.2 percent, the company said.

Pharmacy same-store sales were up 8.3 percent, but up only 4.1 percent excluding the extra day.

Walgreen had been hit by a slow start to the flu season this winter. While more flu cases were reported during February, flu-related prescriptions were below the year-earlier level, Walgreen said.

Pharmacy sales were also hurt by the switch of allergy medicine Zyrtec to over-the-counter status, Walgreen said, although that switch helped "front end" sales, or sales of general merchandise.

Front-end same-store sales, were up 8.2 percent, the company said. Front-end same-store sales were up 4.6 percent excluding the extra day.

Walgreen shares closed at $36.45 on Monday on the New York Stock Exchange. The stock is down 4 percent this year, while shares of rival CVS Caremark Corp (CVS.N: Quote, Profile, Research) is up about 1 percent.

(Reporting by Brad Dorfman, editing by Gerald E. McCormick and Derek Caney)