END OF THE LINE: The lethal injection table at San Quentin State Prison in California.

An anaesthetic used in lethal injections will no longer be made by its only US manufacturer because the company does not want it to be used in executions, forcing states that allow the death penalty to look for other suppliers.

Hospira said on Friday that sodium thiopental has been in short supply for about a year because of manufacturing problems.

The company was planning to shift production to its plant in Liscate, Italy, but the Italian parliament will only allow the drug to be made there if Hospira can guarantee that it will not be used in capital punishment.

Italy is a member of the European Union, which has banned the death penalty and criticised the United States for allowing it.

Sodium thiopental is the first of a sequence of three drugs administered in US lethal injections that paralyse breathing and stop the heart. A sedative is legally required in all lethal injections of US death row inmates.

"This is not how the drug is intended to be used," Tareta Adams, a spokeswoman for Hospira, said in a telephone interview. "We've decided we're no longer going to work to bring the drug back."

Adams said Hospira typically distributes the drug through wholesalers, making it difficult to guarantee that it will not end up in the hands of US correctional authorities.

At least two US states that execute inmates through lethal injection have already tried to import sodium thiopental from a British company, the name of which has not been disclosed. London-based human rights group Reprieve sued the British government in November to stop export of the drug.

Texas, one of the United States' biggest users of the lethal drug combination, is looking for alternative drugs, according to Jason Clark, a spokesman for the Texas Department of Criminal Justice.

Clark said four executions had already been scheduled for this year - two for February, one for May and one for July.

"At this time, we have enough sodium thiopental on hand to carry out the two executions scheduled for February. In March, our supply of this particular drug is set to expire," Clark said in an emailed response.

Texas has 317 men and women on death row, he said.

Hospira, based in Lake Forest, Illinois, said that halting production of the drug would mean hospital customers using it for medical purposes would be unable to obtain it.

"We've been working to get it back because of the medical use," Adams said.

The drug is an older anaesthetic and many doctors who used it for medical purposes have turned to substitutes because of its scarcity.

Oklahoma executed a man in December using pentobarbital, a drug used to euthanise animals, and as a sedative for humans.

A majority of US states have the death penalty, with lethal injection being the primary method of execution in most cases.