(CNN) Lethal injections will probably become more difficult to carry out in the United States because Pfizer, the second-largest pharmaceutical company in the world, has strengthened controls to prevent its products from being used in executions.
Drugs used in executions are no longer available through "the normal pharmaceutical market," said Robert Dunham, executive director of the Death Penalty Information Center. Every U.S. drug maker with products that have been used in executions has gone on record to say they don't want that to happen any more, he said.
"States at this point have decisions to make," Dunham said.
Lethal injection is the primary means of execution in all 31 death-penalty states. Since 1976, 1,436 people have been executed in the United States -- all but 175 by lethal injection, according to the DPIC.
With supplies of lethal injection drugs running low and new sources increasingly difficult to come by, states are grappling with alternatives. Several executions have been delayed recently.
Pfizer spokesman Dean Mastrojohn said Friday that the company is "enhancing the controls on wholesalers and distributors and establishing a surveillance and monitoring system to assess compliance with our policy."
A statement issued by the company said:
"Pfizer makes its products solely to enhance and save the lives of the patients we serve. We strongly object to the use of any of our products in the lethal injection process for capital punishment.
"We are committed to ensuring that our products remain available and accessible to the medical professionals and patients who rely upon them every day. We have implemented a comprehensive strategy and enhanced restricted distribution protocols for a select group of products to help combat their unauthorized use for capital punishment."
The development was first reported by The New York Times.
Megan McCracken, a death penalty expert from University of California, Berkeley, said the company's decision is significant.
"Pfizer is the second-biggest pharmaceutical company in the world," she said. "From the stance of a company making a stand it's a big deal. It's more than a PR move, because they've put in place a restrictive distribution system."
McCracken said it's difficult to determine how much impact the Pfizer decision will have because of secrecy on the state level about lethal injection drugs.
"It is difficult if not impossible to know what (drugs) states are using and how they are getting them," she said. "And if they are using what they claim."
Texas carries out the most executions. Texas Department of Criminal Justice spokesman Robert Hurst told CNN they were aware of media reports of the Pfizer move, but have no comment.
Lethal injection initially required a three-drug cocktail: The first (sodium thiopental or pentobarbital) puts the prisoner to sleep, the second (pancuronium bromide) brings on paralysis, and the final agent (potassium chloride) stops the heart.
In 2010, European drug manufacturers began to ban exports of the cocktail ingredients to the United States. The following year, concerned about the use of sodium thiopental in executions, Illinois-based Hospira stopped making the drug.
Pfizer acquired Hospira last year. The policy posted on the Pfizer website on Friday was an update of the existing Hospira policy, Mastrojohn said.
Denmark-based Lundbeck banned U.S. prisons from using its pentobarbital.
The United Kingdom also introduced a ban on exporting sodium thiopental, and the European Union took an official stance in 2012 with its Regulation on Products used for Capital Punishment and Torture.
Death penalty states began looking for alternatives.
How to Kill: America's death penalty dilemma
Among them: procuring the drugs from alternative sources, devising a one-drug method, employing other drugs such as midazolam or propofol, and using controversial compounding pharmacies to manufacture the drugs.
A number of lawsuits have been filed alleging lethal injections using these drugs caused undue suffering.
In 2014, numerous executions, all employing midazolam, were widely considered botched. In Ohio, Dennis McGuire gasped and convulsed for 10 minutes before dying. In Arizona, Joseph Wood snorted and gulped for air as he died over a period of two hours. And in Oklahoma, Clayton Lockett writhed for 43 minutes before succumbing to a heart attack.
Dunham said some states are still carrying out lethal injection executions using impure drugs from compounding pharmacies, which don't face the same regulations that major pharmaceutical companies do.
Because states don't reveal everything about executions, companies such as Pfizer may not even know if their distribution contracts have been violated, he said.
"States have to decide: Are they going to try to break the law in order to carry out executions, are they going to rely on questionable compounding sources or are they going to change their method of executions or abandon the death penalty altogether?" he said.