Drug companies are working to develop a pure, more powerful version of the nation's second most-abused medicine, which has addiction experts worried that it could spur a new wave of abuse. The new pills contain the highly addictive painkiller hydrocodone, packing up to 10 times the amount of the drug as existing medications such as Vicodin. Four companies have begun patient testing, and one of them ? Zogenix of San Diego ? plans to apply early next year to begin marketing its product, Zohydro. 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.