Dow Jones Newswires | Walgreen Co. posted a surprise 1.1 percent  drop in same-store sales for January as the drugstore giant saw weakness that emerged in December continue.

The company blamed both calendar impacts and fewer incidences of flu for the results.

Walgreen said January same-store pharmacy sales fell 1.2 percent, cut by 2.3 percentage points from new generic drugs hitting the market the past year and fewer cases of the flu. The company said results were hurt by 1.9 percentage points from the month having one less Thursday and one more Sunday versus January 2009.

Prescriptions filled grew 2.7 percent on a same-store basis.

Walgreen’s sales in the front end of stores open at least a year declined 1 percent .

Analysts surveyed by Thomson Reuters expected same-store sales to rise 2.2 percent , with a 3.2 percent  rise in the pharmacy more than offsetting a 0.1% drop at the front end.

Demand for flu vaccines boosted both general-merchandise and pharmacy sales in the fall, although it didn’t help results in December.

On Thursday, Rite Aid Corp. posted a 2.1 percent  same-store sales decline in January, as the drugstore chain continued to struggle with weak front-of-store sales. CVS Caremark Corp. , a key competitor of Walgreen, doesn’t typically release monthly sales results.