Texas prison officials are changing the way they do executions due to a drug shortage.

Texas has been using the sedative pentobarbital in combination with two other drugs.

But the Texas Department of Criminal Justice said in a statement to The Associated Press on Tuesday that it will now use pentobarbital by itself.

Department spokesman Jason Clark says the agency changed its protocol Monday because its stock of one of the other drugs expired.

Pentobarbital also is in short supply as its maker stopped selling it for executions last year. Texas officials said in May that the state had enough doses for 23 executions.

Texas began using pentobarbital last year to replace sodium thiopental, which became unavailable when its European supplier stopped making it under pressure from death penalty opponents.