(Adds Zogenix comment, updates share price)

By Susan Heavey

WASHINGTON, March 28 (Reuters) - Officials in Massachusetts have blocked sales of Zogenix's controversial but U.S.-approved painkiller Zohydro, prompting the drugmaker to criticize what it called an "unprecedented action."

The state's ban "only serves to unfairly restrict patient access," the company said in a statement late Thursday. "Ultimately, the ban on the prescription medication will add to patient suffering in the state," it added.

Massachusetts Governor Deval Patrick announced the Zohydro ban in a speech on Thursday, formally declaring a public health emergency stemming from the abuse of opioids in the New England state.

It is the latest salvo in the ongoing battle over Zohydro's launch as the United States grapples with a wave of abuse of opioids, a class of drugs that includes not only powerful prescription painkillers but also heroin.

On Friday, shares of the San Diego, California-based company were down 6.7 percent in afternoon trade on the Nasdaq.

Bradley Galer, chief medical officer of Zogenix, said Massachusetts officials are misguided about Zohydro's potency and dosage, and noted that other painkillers without abuse-resistant technologies are already on the U.S. market.

"It's not unusual whatsover," Galer told Reuters in an interview on Friday.

The federal Food and Drug Administration approved Zohydro last year over the objections of its advisory panel, which expressed concerns about the potential for abuse.

The drug has since come under further scrutiny from members of Congress, dozens of state attorney generals, medical groups and drug treatment experts seeking to block the drug even as the FDA's top official has defended its action.

While the FDA approves drugs for sale in the United States, it does not guarantee their availability. For example, health insurance companies can exclude certain medications from their formularies.

Zohydro is a extended-release form of hydrocodone that, unlike rival products such as AbbVie Inc's Vicodin or UCB's Lortab, does not contain acetaminophen.

The company has defended the drug as a necessary option for pain patients who cannot tolerate acetaminophen, a nonsteroidal anti-inflammatory drug linked to liver damage and stomach bleeding.

But critics worry that with no built-in abuse deterrents, Zohydro will be a draw for addicts looking for an easy fix.

Massachusetts' Patrick, in his speech, called Zohydro "a dangerously addictive pharmaceutical painkiller, and said it will not be sold in his state "until it is available in an abuse-deterrent form." He also supported calls for the FDA to reverse its approval of the drug and said he has written members of Congress and the Department of Health and human Services, which oversees the FDA.

The fate of Zohydro is key for Zogenix, which has only one other FDA-approved product, although it is developing others. Rival Purdue Pharma has said it is working on a hydrocodone-only drug that will be resistant to abuse, something Zogenix has also pledged to do.

"Once/if the Purdue product is approved, possibly in 2015, FDA could then rule that Zohydro is not as safe as the abuse-deterrent product and either pull the drug's approval or ask Zogenix to withdraw it voluntarily," Rob Smith, an analyst with Capital Alpha Partners, said earlier this week.

Galer said Zogenix is "not concerned" about the competition.

Zogenix, in its statement, said it looked forward to reviewing "the safe use measures already in place" for Zohydro with Massachusetts officials, although Galer said no meeting had been set yet. (Reporting by Susan Heavey; Editing by Bernard Orr)