While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Walgreen Company (NASDAQ:WBA) slipped about 1% today after Cantor Fitzgerald downgraded the drugstore chain from hold to sell.

So what: Along with the downgrade, analyst Ajay Jain planted a price target of $50 on the stock, representing about 25% worth of downside to yesterday's close. While momentum traders might be attracted to the stock's strength over the past year, Jain's call could reflect a growing sense on Wall Street that Walgreen's run is overdone. 

Now what: According to Cantor, Walgreen's risk/reward trade-off is pretty unattractive at this point. "[W]ith WAG's share price up by close to 17% following the 1Q earnings release on 12/20, we can't justify a HOLD rating without a meaningful increase in our price target," Jain said. "WAG also discounts significant earnings upside from Alliance Boots even as the core earnings growth remains in the low-single digits. Finally, based on what we view as some questionable earnings quality recently, we think it's appropriate to highlight this issue." When you couple those seemingly reasonable concerns with Walgreen's 20-plus P/E, it's tough to disagree with Cantor's bearishness.