Pharmaceutical investors often forget that places beyond the United States and the European Union have their own regulatory bodies that approve drugs -- and that some of these markets can be quite lucrative. Canada, Japan, and Australia are home to some of the biggest economies in the world, and they all have their own independent drug-regulation entities.

Smartly realizing the opportunity for sales growth, biopharmaceutical company Elan (NYSE:ELN) and partner Biogen Idec (NASDAQ:BIIB) have been diligently attempting to gain approval for the companies' multiple sclerosis product, Tysabri, in some of those other parts of the world. Yesterday, they announced a bit of success -- Canada's drug-regulatory agency has approved Tysabri to treat relapsing-remitting MS, the most common subtype of the disease.

To put this in perspective, Canada has possibly the highest rate of MS in the whole world. An estimated 55,000 to 75,000 Canadians suffer from the disease -- that's nearly one-fifth the estimated number of patients in the United States.

The fortunes of Elan will rise and fall with the prospects of Tysabri. When the drug was taken off the market early last year, many investors pegged the company for bankruptcy because of its high debt load. Now that the drug has been approved for sale again in the U.S. and also for marketing in the EU, Elan has a chance to start making enough money to stem the company's losses and hopefully begin paring down some of the nearly $2 billion in debt that will be due in varying amounts in 2008 and 2011. One thing that's important to remember is that Elan doesn't get to keep all of the revenues from Tysabri. Its partner, Biogen Idec, gets its cut, too.

Next month, when Biogen and Elan report third-quarter results, investors will get a preliminary indication of how these sales of Tysabri are ramping up. Obviously, gaining approval to sell a drug in Canada and the other fragmented markets throughout the world can be an expensive and lengthy process, given that each country has its own sets of rules and regulations and that an independent sales force is a requirement. Nevertheless, sales in these countries can provide years of sales growth. Investors in Elan shouldn't forget that.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.