Novartis and its Sandoz unit, maker of a generic version of an anaesthetic used in lethal injections in the United States, have taken steps to try to stop the drug ending up in the United States.

"Sandoz has also advised all of its subsidiaries with locally approved marketing authorisations for sodium thiopental to not sell the product to distributors or third parties that may be selling it into the U.S.," Novartis and Sandoz said in a statement.

Last month, U.S. specialty medicines maker Hospira said it was halting its production of sodium thiopental as it did not want it to be used in executions.

Hospira said it 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.

"Sandoz and Novartis support only the authorized use of injectable thiopental, which is primarily indicated for the induction of anaesthesia, and do not support the sale of this or any product for use in non-approved treatments," Novartis and Sandoz said in a statement.

Sandoz makes injectable thiopental under contract for a third party located in the UK, which sells it directly to Archimedes Pharma. The British group is responsible for the product's marketing and commercial supply under its respective UK marketing authorisation, Novartis and Sandoz said in the statement.

Novartis and Sandoz also said Sandoz does not market the drug in the United States or ship or sell directly to any third party selling this product into the United States.

Archimedes has never exported the product directly into the United States, Deborah Saw, a spokeswoman for the group said. It sells the drug to a distributor, which then sells it to hospital pharmacies, primarily in Britain's National Health Service, and also to other wholesalers. Archimedes does not have information on specific end-purchasers or users of its products, she said.

Sandoz and Novartis also said Sandoz was not aware of, and not able to monitor or control, the supply chain beyond its own direct customers, as it was not responsible or involved in the marketing and commercial activities of third parties.

Last November, activists sued the British government to stop the export of the drug used in capital punishment in the United States, but Business Secretary Vince Cable said he would not issue a ban because the drug can be used for legitimate purposes.

Sodium thiopental, a sedative legally required for U.S. lethal injections, is in short supply in the United States, and at least one U.S. state has already turned to Britain to fill the gap.