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Drug makers make unlikely candidates for the next big short. Kyle Bass, a hedge fund manager who bet successfully on a housing market crash in the United States, is aiming at pharmaceutical companies worth a combined $450 billion in market capitalization.

A new method of challenging patents gives him a tool. But it’s not clear how his firm, Hayman Capital Management, has an advantage over existing, aggressive and experienced generic firms.

Mr. Bass, based in Dallas, has already drawn blood. Acorda Therapeutics stock fell almost 10 percent on news of his challenge on Tuesday. The biotech company, with a $1.5 billion market capitalization, gets nearly all its revenue from Ampyra, a treatment for multiple sclerosis. If cut-price competition emerges sooner than expected, the company would be hurt badly.

Hayman is using a newish process called “inter partes review.” If the United States Patent Office accepts the petition, one of Acorda’s patents may be disallowed within the next year for the obviousness of its technology. That’s possible, because the crucial molecule in its treatment was used for decades as bird poison. Acorda figured out how to modify the drug, and use it safely and effectively.

Mr. Bass isn’t alone in going after Acorda. Eight generic firms are challenging Ampyra’s patents in court. Instead of betting on Acorda’s stock falling, they want to introduce their own cut-price versions of the drug. There are big profits for the generic company that wins — rivals have to wait six months to start selling their own copycats.

The hedge fund’s strategy has a few pluses. Generic drug firms are reluctant to use inter partes review because they want the six months of exclusivity from a successful court challenge. It’s possible the patent office will be more likely to overturn patents.

Yet the market isn’t obviously inefficient. Generic firms have plenty of capital, incentive and experience to take on drug companies. Hayman’s inter partes review tool is also limited. The patent agency will only examine claims of obviousness. Even if Mr. Bass succeeds against Acorda, the company’s drug will still have four patents protecting it against generic competition until 2027. This all explains why Acorda has since regained about a third of its initial losses.

Mr. Bass’s initial success will surely encourage more of these challenges, though. Companies with blockbuster, high-price therapies developed from molecules discovered decades ago, including Celgene and Biogen Idec — each worth more than $90 billion — are probably next in his line of fire.

Robert Cyran is a columnist for Reuters Breakingviews. For more independent commentary and analysis, visit breakingviews.com.