Canadian health officials watched closely on Friday as Alli, the first over-the-counter diet pill approved by the U.S. Food and Drug Administration, went on sale south of the border.

The drug, which reduces the amount of fat absorbed by the body after a meal by roughly 25 per cent, is available in Canada under a different name, but still requires a prescription.

"It's been around for quite a long time," Yoni Freedhoff of the Bariatric Medical Institute, an Ottawa-based weight loss centre, told CTV News.

"Studies show that people taking it for two years lose up to eight kilograms. But the way it works, of course, is by either delaying or blocking the absorption of fat in the gut. And the way it blocks it is, well, it comes out -- 'fast' is a good objective to use."

Alli's manufacturer, GlaxoSmithKline, has warned the pill should be used in conjunction with dieting and exercise.

In the U.S., it costs $60 dollars for a month's supply of 90 capsules. But some doctors say it's not an easy solution for obesity.

Dr. Timothy Johnson, ABC News medical editor, said the pill can have some unpleasant side-effects if used incorrectly.

"If you take too much of the pill, or eat too much fat with the pill, it will cause a (gastrointestinal) disturbance: gas diarrhea," said Johnson.

"In fact, the company says when you start the pill, you should do so on a weekend and stay close to home. What they mean is, stay close to the bathroom, to be honest."

In clinical trials, about half of patients reported gastrointestinal side effects, including leakages and oily discharges.

But Freedhoff said the drug also urges people to pay more intention to the amount of fat in their diet, which can be another tool in fighting obesity.

"The folks who remain on the drug are able to do so by learning which foods have fat in them, by monitoring the side-effects (of the pill)," he said.

"Frankly, I'd much rather be looking at food labels and reading about the fat content in my food, if that was what I was trying to minimize, other than parking myself close to a bathroom and hoping for the best."

With a report by CTV's Rosemary Thompson in Ottawa