WASHINGTON — Faced with a national shortage of a key drug used for lethal injections, Oklahoma now hopes to turn to an anesthetic used to put down animals and is awaiting a court ruling on the issue.

For months now, several US states have struggled to find supplies of sodium thiopental -- the first and most crucial of three drugs used in lethal injections. The shortage has forced some states to put executions on hold.

"The reason the first drug matters so much is because... if properly administered, it will put the inmate in a state of unconsciousness where he does not feel the second and third drug," University of California, Berkeley professor Elisabeth Semel told AFP.

"If you use a different drug, an unknown drug, then you don't know whether or not it has been mixed differently, whether or not the staff has the expertise to administer it properly."

The only US pharmaceutical company that manufactures sodium thiopental, Hospira, is currently out of stock and will not be able to resume production until the first quarter of 2011.

And Hospira's most recent batch is nearing its 2011 expiration date.

Some states like Texas have enough thiopental to press ahead with their death row schedules, but others like Kentucky have been forced to put executions on hold.

On October 27, Arizona executed 48-year-old Jeffrey Landrigan for a 1989 murder after importing the anesthetic from an undisclosed foreign manufacturer authorized by the US Supreme Court.

Oklahoma put to death Donald Wickerly, 41, on October 14 after obtaining a dose of sodium thiopental from neighboring Arkansas state.

Now on December 16, Oklahoma plans to execute convicted murderer John Duty, 58, using the animal anaesthetic pentobarbital, employed by veterinarians to put animals to sleep.

Duty's lawyers in a court document worry their client will be used as a "guinea pig" to test this new method of execution.

"In the absence of expert evidence, clinical data, or other similarly sound application of scientific methods providing reliable information of expected outcomes, Mr Duty will be a human subject for an unproven mechanism of execution," they said.

"The Supreme Court has acknowledged that the use of other drugs would be torturous if there is any problem with the first drug," said capital punishment specialist Megan McCraken, referring to a 2008 decision.

Semel said the first anesthetic was key to the whole procedure, as "it's used to render the inmate unconscious so that he will not suffer excruciating pain from the drug that is used to kill him, the last drug."

The issue "becomes one of transparency," Jen Moreno, another legal expert, told AFP.

"The state shouldn't be allowed to just switch drug in secret, without any sort of inquiry into whether or not it's going to perform as intended," she said.

In its response, Oklahoma has stressed that pentobarbital "widely used by veterinarians for animal euthanasia... has certainly been used as an anesthetic before (on humans) and is hardly experimental."

Federal judge Stephen Friot will hold a hearing Friday to review the current standing and information on pentobarbital before deciding whether Oklahoma should be allowed to modify its execution protocol with a new product.

"Why don't they wait until January when the (old) drug will be available?" argued Semel. "Because they want to carry out the execution."

She said the state was setting a cavalier example, with "the kind of 'trust us' attitude, 'we know what we're doing and we should simply be allowed to do this behind close doors as we always have.'"

"If they have an execution date, they want to go forward, they do not want to stand in the way. And so, if they think it can find another drug that would be a substitute, then they are going to go to court and argue they should be able to use it."