Two phrases that drug companies almost never want to see in the same sentence: “clinical trial” and “side effects.” Almost any time an experimental drug has an effect other than the intended one—treating heart disease, diabetes, whatever—it’s bad news, which is why the huge majority of drugs flame out in clinical trials and never reach the market (or get pulled from the market, as Vioxx was when it was found to raise patients’ risk of heart attacks).

But every once in a great while, a side effect offers new possible uses for a drug. The most famous example is minoxidil—better known as Rogaine—which opens up blood vessels and was initially developed to treat high blood pressure. But when men began developing an interesting side effect—growing hair—the manufacturer (Upjohn, whose remnants have since become part of Pfizer) saw whole new possibilities, and the rest is hirsute history.

Now Allergan, Inc., best known as the maker of Botox (though it also makes Lap-Band, placed around a patient’s stomach as a treatment for obesity), may be in the same happy position, and Maybelline might want to look over its shoulder: a glaucoma drug that Allergan sells under the name Lumigan turns out to grow eyelashes that a model would kill for. At a conference in New York this week, the company announced that it would seek approval for this use of Lumigan by the end of the third quarter of 2008.

The company suspected it might have a hit on its hands when “no one in the clinical trial” returned the unused portion of the drug, Allergan’s head of R&D, Scott Whitcup, told me when he dropped by this morning. The company expects approval for this use—the drug would be applied to the base of the lashes, with each application lasting maybe three or four weeks—by next year.

The pharmaceutical industry has good reason for taking such side effects seriously, rather than abandoning a drug that does unexpected things. Although pharma R&D spending has soared past $40 billion a year (inventing, developing and testing a new drug costs something north of $800 million, if you include all the busts), the number of new FDA approvals has stagnated. “More and more,” said Whitcup, “people are realizing that ‘side effects’ might be effects,” as in something a company can market a drug for.”

Lush eyelashes aren’t exactly a cure for cancer, but the principle is a sound one: these days, drug companies can't afford to let any biological effect of a compound go unexplored.