A Danish drug maker moved on Friday to curb the increasing use of one of its sedatives in U.S. executions by requiring distributors to sign an agreement that they won\'t sell it for that purpose. The change represents yet another obstacle for states that have struggled recently to revamp their lethal injections. Pentobarbital manufacturer Lundbeck Inc. said it will require that U.S. drug suppliers vow not to sell it to prisons that plan to use it for executions, and any supplier that breaks the agreement will face court action. About two-thirds of the 34 death penalty states have switched to the drug or are considering as a replacement for another drug that\'s no longer for sale here. Many states said they have stockpiled pentobarbital and that Lundbeck\'s move won\'t immediately affect their execution plans. But defense attorneys and legal experts said the change will make it hard for them to replenish their supplies, which could kick off the complicated process of finding another replacement. Article continues below \"States may well be already looking for another alternative - otherwise they won\'t be able to perform executions,\" said Deborah Denno, a Fordham University law professor. Pentobarbital, which is produced in Lundbeck\'s plant in Kansas, is also used to treat seizures and destroy animals. But it has fast become a key part of executions since the sole U.S. manufacturer of sodium thiopental, once an execution mainstay, announced this year that it would not resume production of it. The dwindling supply of sodium thiopental left corrections officials scrambling, and many executions were delayed. They hoped pentobarbital would ease the crisis. In many states, making a switch requires a lengthy regulatory and review process. And any change typically leads to lawsuits from inmates who claim the substance violates the ban on cruel and unusual punishment. Lawsuits over pentobarbital are still being heard. Oklahoma became the first state to use pentobarbital last year, and Ohio, Texas, Georgia and other busy death penalty states soon followed suit. Lundbeck, meanwhile, began trying to stop prisons from using pentobarbital almost as soon as they began adopting it. First, the company said in a statement that using the drug for executions \"goes against everything we\'re in business to do.\" Then it wrote letters to prison officials and politicians saying that using the drug to carry out the death penalty \"falls outside its approved indications.\" The warnings didn\'t have much of an effect as prisons in eight states used the drug to carry out 17 executions this year. Lundbeck hopes this move will halt the trend. Chief executive Ulf Wiinberg said the new policy should bar U.S. prisons from buying the drug indirectly from suppliers, and he warned that the company would take legal action against any distributors who break the agreement. \"Lundbeck will have to approve each order, and everyone buying the product must sign a paper stating they will not sell it on to prisons,\" Wiinberg said. The shift came after months of pressure from anti-death penalty activists and opposition in Lundbeck\'s home country of Denmark, which opposes capital punishment. Maya Foa, an investigator with London-based human rights group Reprieve, said Friday\'s announcement gives Lundbeck more control over what happens to the drug after it\'s sold. Now, she said, \"there aren\'t legitimate channels for the prisons to access the drugs.\" The move won\'t immediately choke off the supply. Several death penalty states have already purchased enough pentobarbital to keep executions running. Texas, for one, has enough of the drug for eight executions it has scheduled through September, said spokesman Jason Clark. And Georgia prisons spokeswoman Kristen Stancil said the state has an \"adequate supply\" to carry out lethal injections. Oklahoma, meanwhile, is in a trickier situation. It has no executions scheduled but also no supply of the drug, said prisons spokesman Jerry Massie. \"It\'s too early for us to make any assessment of what kind of issues this is going to create,\" he said. The stockpiles also have a limited shelf life. A typical expiration date for injected drugs such as pentobarbital is one to two years, said Erin Fox, manager of the University of Utah\'s Drug Information Service. It\'s unclear whether suppliers who already have the drug will be restricted from selling more of it. Cardinal Health, an Ohio-based pharmaceutical distributor which sold Georgia and Oregon their supplies, said it is working with Lundbeck to implement the new strategy. And it\'s impossible to guarantee that vendors won\'t still try to sell the chemical to state prisons through backchannels. Drugs are often sold from one distributor to another, meaning they can pass through several hands between the manufacturer and end-user. The move comes after unusual circumstances surrounding two recent executions using pentobarbital raised concerns among death penalty experts. Eddie Duval Powell raised his head with a confused look on his faced and glanced around Alabama\'s death chamber after he was injected with the drug on June 16. And Roy Willard Blankenship jerked his head several times and appeared to gasp for air last week after Georgia officials used the drug to execute him. Brian Kammer, Blankenship\'s defense attorney, unsuccessfully urged state and federal courts to halt the execution on grounds that pentobarbital is an unreliable drug for executions, and that it would cause his client to needlessly suffer. He said Friday that Lundbeck\'s new requirement should make it \"close to impossible\" for prisons to get the drug - at least until they find an alternate drug. \"For now, I think this means that once corrections agencies run out of their current supplies of pentobarbital, there will be a drug supply crisis in terms of implementing capital punishment,\" he said.