The US state of Texas is running out of a sedative used for lethal injections for the second time in 13 months.
Texas Department of Criminal Justice spokesman Jason Clark confirmed pentobarbital, which has been used in executions in the state since July 2012, will reach its expiration date in September.
Texas, which has a higher execution rate than any other US state, switched to pentobarbital when supplies of sodium thiopental 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 euthanise pets and other animals.
Texas has executed 11 inmates so far in 2013, including one on Wednesday night, while ten executions have been carried out elsewhere in the country.
Officials said there are five more Texas executions scheduled for this year, with the next one set for 19 September.
Mr 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.
Mr Dieter said all executions carried out in the US 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 and has been used in executions by the state of Missouri.
But this drug raises a similar problem.
The British-based company that markets it, AstraZeneca, has said that after the Missouri execution it will not allow any of its products to be exported for use in capital punishment.
States might consider turning to compounding pharmacies, small laboratories that can remix existing narcotics to fill specific needs, generally on the order of a doctor, Mr 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, which is more than any other state since a 1976 US Supreme Court ruling affirming capital punishment.