NEW YORK — Drugmakers 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 — 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.

Critics say they are especially worried about Zohydro, a timed-release drug meant for managing moderate to severe pain, because abusers could crush it to release an intense, immediate high.

“I have a big concern that this could be the next OxyContin,” said April Rovero, head of the National Coalition Against Prescription Drug Abuse. “We just don’t need this on the market.”

OxyContin, introduced in 1995 by Purdue Pharma of Stamford, Conn., was designed to manage pain with a formula that dribbled one dose of oxycodone over many hours.

Abusers quickly discovered they could defeat the timed-release feature by crushing the pills. Purdue Pharma changed the formula to make OxyContin more tamper-resistant, but addicts have moved on to generic oxycodone and other drugs that have no timed-release feature.

Oxycodone is now the most-abused medicine in the United States, with hydrocodone second, according to the Drug Enforcement Administration’s annual count of drug seizures sent to police drug labs for analysis.

The latest drug tests come as more drugmakers are getting into the $10 billion-a-year legal market for powerful — and addictive — opiate narcotics.

“The whole supply-side system is set up to perpetuate this massive unloading of opioid narcotics on the American public,” said Peter Jackson, co-founder of Advocates for the Reform of Prescription Opioids.

The pharmaceutical firms say the new hydrocodone drugs give doctors another tool to try on patients in legitimate pain.

“Sometimes you circulate a patient between various opioids, and some may have a better effect than others,” said Karsten Lindhardt, chief executive of Egalet, which is testing its own pure hydrocodone product.

The companies say a pure hydrocodone pill would avoid liver problems linked to high doses of acetaminophen, an ingredient in products like Vicodin. They also say patients will be more closely supervised because, by law, they will have to return to their doctors each time they need more pills. Prescriptions for the weaker, hydrocodone-acetaminophen products now on the market can be refilled up to five times.

Zogenix, of San Diego, has finished three rounds of patient testing, and last week it announced it had held a final meeting with Food and Drug Administration officials to talk about its upcoming drug application. It plans to file the application in early 2012 and have Zohydro on the market by early 2013.

Purdue Pharma and Cephalon, a Frazer, Pa.-based unit of Israel-based Teva Pharmaceuticals, are conducting late-stage trials of their own hydrocodone drugs, according to documents filed with federal regulators. In May, Purdue Pharma received a patent applying extended-release technology to hydrocodone.

Meanwhile, Denmark-based Egalet has finished the most preliminary stages of testing aimed at determining a drug’s basic safety. The company could have a product on the market as early as 2015 but wants to see how the other firms fare with the FDA before deciding whether to move forward, Lindhardt said.

Critics say they are troubled by the dark side of the boom in narcotic painkillers: murders, pharmacy robberies and millions of dollars lost by hospitals that must treat overdose victims.

Thousands of legitimate pain patients are becoming addicted to prescription painkillers, they say, in addition to the thousands more who abuse the drugs.

Prescription painkillers led to almost 15,000 deaths in 2008, compared to 4,000 in 1999, the Centers for Disease Control and Prevention reported last month.

Emergency room visits related to hydrocodone abuse have shot from 19,221 in 2000 to 86,258 in 2009, according to data compiled by the Drug Enforcement Administration. In Florida alone, hydrocodone caused 910 deaths and contributed to 1,803 others between 2003 and 2007.

FDA spokeswoman Erica Jefferson said the agency would not comment on its discussions with drug companies, citing the need to protect trade secrets.

The DEA, which enforces controls on medicines along with the FDA, said it could not comment on drugs that have not yet been approved for sale.