(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.

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