The company also saw sales and earnings pressured because 2009 had one less day in the quarter than 2008, which was a leap year.
Walgreen posted second-quarter profit of $640 million, or 65 cents per share, in the fiscal second quarter that ended February 28, compared with a profit of $686 million, or 69 cents per share, a year earlier.
In the past, drugstores could typically charge more than chains such as Wal-Mart Stores Inc
Walgreen shares traded at $25.45 on Monday in premarket trading, up from Friday's New York Stock Exchange close of $24.29.
(Reporting by Brad Dorfman and Jessica Wohl; Editing by Maureen Bavdek)