Walgreen's revenue from established stores climbed 3.7 percent last month but missed analyst expectations, as prescription sales for the nation's largest drugstore chain also fell short of the average forecast on Wall Street.

The Deerfield, Illinois, company said Thursday that it saw 5 percent growth in pharmacy sales from stores that have been open at least a year and a 1.4 percent gain from the front end, or the rest of the store.

Analysts expected 5.5 percent growth in pharmacy sales, a 1 percent gain from the front end, and a 3.8 percent increase overall, according to Thomson Reuters.

Revenue from stores open at least a year is a key indicator of a retailer's health, because it excludes the impact of recently opened or closed stores.

Walgreen Co. ran 8,206 drugstores at the end of the month, 91 more than a year ago. It said customer traffic to its stores fell 1.7 percent last month, but basket size, or the amount they purchased, climbed about 3 percent.

Walgreen said a shifting calendar hurt sales comparisons because last month contained one additional Sunday and one fewer Thursday than August 2013. Drugstores typically do more business on weekdays, when customers visit to fill prescriptions after seeing their doctors.

Walgreen also said total revenue from its fiscal fourth quarter climbed 6.2 percent to $19.06 billion. The company will report full results from that quarter on Sept. 30.

Company shares slipped 46 cents to $59.76 before markets opened Thursday and after Walgreen announced August sales. The stock had climbed about 5 percent so far this year, as of Wednesday's market close. The Standard & Poor's 500 index, in contrast, has risen 8.2 percent.