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Lenny Ignelzi, Associated Press
In this Dec. 22, 2011 photo, the sun slowly sets on the empty parking lot at the Zogenix headquarters in San Diego. The pharmaceutical maker is one of at least four companies working on purer, more powerful versions of the nation’s second most-abused medicine, hydrocodone, a trend that has addiction experts worried that it could spur a new round of abuse.

NEW YORK — Drug companies are working to develop a pure, more powerful version of the nation's second most-abused medicine. That has addiction experts worried that it could spur a new wave of abuse.

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The new pills contain the addictive painkiller hydrocodone. They pack up to 10 times the amount of the drug as medications such as Vicodin.

If approved, it would mark the first time patients could legally buy pure hydrocodone. Existing products combine the drug with nonaddictive painkillers such as acetaminophen.

Critics say abusers might crush the pills to get high. That's exactly what people do with oxycodone, which is the most-abused medicine in the U.S.

The pharmaceutical firms say the new drugs give doctors another tool to help patients manage pain. The companies also say patients will be more closely supervised.