When GlaxoSmithKline’s new nonprescription fat-blocker, alli, hit the market in Santa Monica, California, one pharmacist said it was like nothing she had ever seen.

“I have never in my life experienced anything like this,” the unspecified pharmacist from Walgreens said in the Wall Street Journal. The pharmacist was referring to the demand for the drug, which emptied drug store shelves within hours of its release.

The drug is an over-the-counter version of Roche’s Xenical, which curbs the absorption of calorie-rich fat in the intestines. The product comes in a 90-pill starter set, which costs $62.99 online.

But the U.S. Food and Drug Administration is warning consumers that the drug may not be entirely pleasant.

“The most common side effect of this product is a change in bowel habits, which may include loose stools,” the FDA said in a statement.

GSK offers some advice to consumers in case unexpected bowels prove too great of a surprise. “You may feel an urgent need to go to the bathroom,” the drug’s Web site warns. “Until you have a sense of any treatment effects, it’s probably a smart idea to wear dark pants, and bring a change of clothes with you to work.”

Sources: Scott Hensley, “Hunt is on for Diet Drug,” Wall Street Journal, June 15, 2007; GlaxoSmithKline, My Alli website.