OKLAHOMA CITY (Reuters) - Oklahoma, which executes more prisoners per capita than any other state, said on Wednesday it has only one remaining dose of pentobarbital, a key drug used to kill condemned prisoners.

One reason the state is running out is because of a ban on the sale of drugs for such purposes by the European Union, which opposes the death penalty.

Oklahoma has a single vial of pentobarbital left after the execution on Tuesday night of 57-year-old Michael B. Selsor, prison spokesman Jerry Massie said.

Oklahoma is the first state to publicly admit it has nearly exhausted supplies of the drug but other states may follow because of the EU clamp down, said Richard Dieter, executive director of the Death Penalty Information Center in Washington, D.C.

Pentobarbital is a sedative that is the first of a three-drug cocktail administered by Oklahoma. It is followed by vecuronium bromide, which stops breathing, and potassium chloride, which stops the heart.

Oklahoma was the first state in the country to use pentobarbital in 2010 after a shortage of another anesthetic, sodium thiopental, caused penal officials in death penalty states to look for an alternative.

Eleven other states also use it. Arizona and Ohio use a single injection of pentobarbital for executions while nine states use the multi-drug protocol, according to the Death Penalty Information Center.

Lundbeck Inc, the only manufacturer of pentobarbital, is located in Denmark and forbids its U.S.-based wholesalers from selling the drug for lethal injections, while the European Union forbids its member countries from exporting drugs for executions.

Oklahoma could resort to another anesthetic never used before in executions, Massie said, or it could try to tap existing supplies of pentobarbital.

A third option, he said, would entail going back to sodium thiopental.

"It's available but you run into the same kind of problem. Companies don't want to use it for executions," Massie said.

The only manufacturer of sodium thiopental in the United States, Hospira Inc, halted production last year.

Dieter said even if states have stockpiled a large supply of pentobarbital, expiration dates eventually will require new orders, he said.

Any change in death penalty procedures typically are met with legal challenges and sometimes lengthy administrative reviews, Dieter added, noting that California has not had an execution since 2006 because of exhaustive review procedures. A measure has qualified for the ballot in November in California calling for repeal of the death penalty.

Oklahoma has executed three men so far this year but has no more executions scheduled. There are 60 people on death row in the state, Massie said.

 The state has the highest number of executions per capita since the death penalty was restored in the United States in 1976. Texas has executed more people but has a far larger population.