Walgreen ( WAG) investors have a bitter pill to swallow -- and they may not be the only ones. Shares of the drug store chain plunged 15% in furious trading Monday after the normally reliable company posted a steep earnings shortfall. And the source of Walgreen's pain -- the once-hot generic drug business -- may spell pain for rivals Medco ( MHS) and CVS ( CVS). "The year-over-year earnings decline is the first that we are aware of -- going back at least 10 years," Bear Stearns analyst Robert Summers wrote. "While it is easy to downplay this miss as one bad quarter for a normally very predictable company, we believe the industry as a whole is heading into a very difficult period as the generic margin benefit begins to roll off dramatically." Walgreen's "significant earnings miss is indicative of tough roads ahead." Specifically, Walgreen reported that earnings dropped 3.8% to $397 million, or 40 cents a share, in the latest quarter. Analysts were expecting profits, which came in at 41 cents a year ago, to climb to 47 cents a share. Walgreen blamed lower payments for popular generic drugs, coupled with higher expenses, for the downturn. Last quarter, for example, Walgreen said that it filled nearly three times as many prescriptions for simvastatin -- the generic version of blockbuster cholesterol drug Zocor -- as it did a year ago. But with prices for simvastatin falling, Walgreen explained, the company's actual gross profits for that drug remained virtually unchanged.