A bottle of the liquid sedative Pentobarbital Sodium. REUTERS/Tomas Bravo SAN ANTONIO (Reuters) - Texas, the state with the highest execution rate in the country, is running out of a sedative used for lethal injections for the second time in 13 months, a state official said on Thursday.

Texas Department of Criminal Justice spokesman Jason Clark confirmed that the state's supply of pentobarbital, which has been used in the execution of condemned killers in the state since July 2012, will reach its expiration date in September.

Texas switched to pentobarbital, a barbiturate that is the drug of choice for physician-assisted suicide in Europe, when supplies of sodium thiopental, previously used by the state in a three-drug execution mix, were cut off.

The Illinois-based pharmaceutical company Hospira Inc said in 2011 it would stop making thiopental after the government of Italy, where it planned to move production of the drug, raised objections, citing its use in executions.

Richard Dieter, who heads the Death Penalty Information Center, said Texas and other states that were forced to switch from thiopental are now facing a problem with the availability of pentobarbital, which is often used to euthanize pets and other animals.

Texas has executed 11 inmates so far in 2013 - including one Wednesday night - while 10 executions have been carried out elsewhere in the country. Officials said five more Texas executions are scheduled for this year, with the next one set for September 19.

Clark said Texas is confident it will be able to continue executions but did not say whether September's execution may have to be delayed.

"Alternate sources of pentobarbital are possible, or an alternate drug," he said.

Dieter said all executions carried out in the United States during the past 13 months have used pentobarbital. He said some states are in the process of obtaining the necessary legal approval to switch to other sedatives.

One option is the surgical anesthetic propofol, which was blamed for the 2009 death of singer Michael Jackson. Missouri revised its execution protocol in 2012 to include it but the state has not had an execution since the protocol was changed.

Makers of the drug have objected to its possible use in executions.

States might consider turning to so-called compounding pharmacies, small laboratories that can remix existing narcotics to fill specific needs, generally on the order of a doctor, Dieter said. But compounding pharmacies might also be reluctant to cooperate on moral grounds.

"They have their own ethics," he said. "I'm not sure that will be a fruitful way to go."

Texas has executed 503 prisoners - more than any other state - since a 1976 U.S. Supreme Court ruling affirming capital punishment.

(Reporting by Jim Forsyth; Editing by Mary Wisniewski and Bill Trott)

(This August 1 story was refiled to correct the first paragraph to reflect Texas now uses single drug in executions)