Pharmaceutical company Pfizer has imposed controls on the distribution of its products to ensure that none are used in lethal injections to execute death row inmates.
The move reportedly closes off the last remaining open-market source of drugs used in executions.
Pfizer said on its website: “We are enforcing a distribution restriction for specific products that have been part of, or considered by some states for, their lethal injection protocols.
“Pfizer strongly objects to the use of its products as lethal injections for capital punishment.”
The New York-based drugmaker said it offers the products because they save or improve lives, and markets them solely for use as indicated in the product labelling.
Reprieve, a human rights organisation opposed to the death penalty, said: “This is a critical turning point in the history of capital punishment in America.
“From today, all FDA-approved manufacturers of all potential execution drugs – a diverse group of 25 global companies – have blocked their sale for use in executions.
“It reflects widespread unease about the procedure, and raises fundamental questions about the administration of the death penalty in America.”
Pfizer said it would restrict the sale of seven products which can be used in executions to a select group of wholesalers.
The wholesalers must certify that they will not resell the drugs to correctional institutions – and will be monitored, the company said.
The list of products includes pancuronium bromide, potassium chloride, idazolam, hydromorphone, rocuronium bromide and vecuronium bromide, and propofol – the drug that caused the death of pop star Michael Jackson.
The New York Times reported that Pfizer was the last major legal source for the drugs used in executions, following similar actions by more than 20 and European drug manufacturers.
Experts say the move will make it far more difficult for US states to carry out lethal injections, but authorities may find other ways of seeking illicit sources of lethal compounds, or else use alternative methods of execution.
Many states have passed laws imposing secrecy on the source of their lethal injection drugs.
There have been 14 executions in the US so far in 2016 in five states: six in Texas, five in Georgia and one each in Alabama, Florida and Missouri. Last year, there were 28 in six states.
Ohio, which last executed an inmate in January 2014, has repeatedly pushed back executions while it looks for drugs. It now has more than two dozen inmates with execution dates, but no drugs to put prisoners to death with.