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Drug store giant Walgreen Co. (NYSE: WAG) announced this morning that December same-store sales fell 6.1% year-over-year, primarily due to the impact of generic drug sales replacing sales of higher-priced brand-name drugs. Total sales fell 4% from $6.99 billion last year to $6.71 billion.

Pharmacy sales in December fell 4.9%, and same-store pharmacy sales fell 8.9%. Pharmacy sales accounted for 56.7% of monthly sales.

Walgreen noted that a calendar shift was responsible for a negative impact of 2.3% on pharmacy sales, following a 2.9% drop in November. Sales of generic drugs as opposed to brand-name drugs cost the store 3.2% in comparable sales.

There was some positive news. The number of flu shots administered this year has risen from 5.3 million a year ago to 5.5 million. An increase in flu this year has boosted same-store drug sales by 1.4%, and the additional flu shots added 0.7% to sales in December.

Walgreen’s stock is down 0.8% this morning, at $37.50 in a 52-week range of $28.53 to $38.41, as virtually every metric reported today was down.