For the month of December, same-store sales at Walgreen Co. declined by 0.3%, which is the first fall recorded since February, mainly because the growth of pharmacy-sales was much weaker as compared to the past few months, as incidence of H1N1 waned. Also, the company was hurt last month by its strategy of reducing holiday inventory.

On the news, shares of the company slipped by 2.4% in pre-market trading. The monthly same-store sales have been much weaker than the estimates pegged by analysts.

Walgreen, however, did confirm that same-store pharmacy sales for December had managed to rise by 1.8%, cut by about 2.1 percentage points on the back of new generic drugs that came out over the past year.

On same-store basis, filing of prescriptions jumped by 4.1%.

"These aren't great numbers, and people already were expecting weak number especially on the front-end. "It raises questions if there's growing pains as the company goes through its rework of stores, and if they are going to have a better balance of taking inventory out of stores versus boosting sales", said Jefferies analyst Scott Mushkin.

With the decline, Walgreen's major transformation project for all its stores, called Customer Centric Retailing, has come under considerable questions and concerns.