Investors in shares of drug companies often think of the Japanese market for pharmaceuticals as an afterthought in comparison to the U.S. and European Union, but it actually represents the second-largest individual market for drug sales in the world, at roughly $60 billion in drug sales in the trailing 12 months. This is roughly twice as big a market as any country in the E.U., and one that grew 7% last year.

Finally taking advantage of this opportunity, last week drug developer Alkermes (NASDAQ:ALKS) announced that its partner, Johnson and Johnson (NYSE:JNJ) subsidiary Janssen, had submitted an application to market its antipsychotic schizophrenia compound Risperdal Consta in Japan.

Risperdal Consta is a longer-lasting version of JNJ's Risperdal, which Alkermes helped to develop. As Alkermes' first approved product and one from which it receives royalties and manufacturing earnings, it accounts for the lion's share of its revenues, at nearly 50% of its $167 million in revenues in fiscal year 2006.

Right now with its two diabetes treatments in phase 3 trials still years away from any possible approval, the biggest near-term driver of shares of Alkermes is going to be how sales Risperdal and its alcohol-dependence treatment, Vivitrol, perform. Revenues from Risperdal are expected to be up anywhere from 19%-27% in fiscal year 2007 compared to the $81 million the drug brought in last year to Alkermes. Vivitrol, though, is getting off to a very slow start with only $10 million in fiscal 2007 sales expected at most. The drug may not be the hit that Alkermes and marketing partner Cephalon (NASDAQ:CEPH) thought it would become.

Hopefully, the entrance of Risperdal into Japan can make up for some of the large downwardly revised sales that Alkermes has forecast in 2007 for Vivitrol (down from an expected $35 million-$40 million in sales). Either way, with Japan commanding over 10% of global pharmaceutical sales and Risperdal sales gaining ground, there will be big bucks for Alkermes if Risperdal can gain marketing approval over there.

Johnson and Johnson is an Income Investor selection.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article.