Texas has nearly exhausted its supply of execution drug

Manufacturers are banning US prisons from using pentobarbital

A lethal injection death chamber in a prison in Huntsville, Texas (Getty)

The state of Texas, where most of the United States' lethal injections take place each year, is running low on pentobarbital, the drug used to carry out executions of death row prisoners.

If the execution of Manuel Vasquez goes ahead as scheduled next Wednesday, the state will have completely run out.

"We're exploring all options," said Jason Clark, spokesman for the Texas Department of Criminal Justice. "Including the continued use of pentobarbital or an alternate drug(s) in the lethal injection process."

States across the US have struggled to maintain a steady supply of the drug recently, after its European manufacturer explicitly banned US prisons from using it in executions.

Last year, Tennesse said that if it can't get hold of pentobarbital it will simply return to electrocuting inmates.

The mooted use of an 'alternate drug' is perhaps even more worrying. In April, a substitute called midazolam used in Oklahoma went wrong and left the inmate writhing and convulsing, taking 43 minutes to die instead of the usual 10.

Texas executed 10 people last year, three so far this year and has the deaths of Randall Mays, Kent Sprouse, Manuel Garza, Richard Vasquez, Robert Pruett, Charles Derrick along with Vasquez scheduled.