A doctor testifying for a death row inmate convicted of killing a police officer 33 years ago testified Tuesday that Florida’s planned use of a replacement drug for lethal injections could cause extreme pain in executions.

Dr. David Waisel, a Boston anesthesiologist, said the drug pentobarbital commonly sold as Nembutal, hasn’t been sufficiently tested to ensure an inmate is unconscious before deadly drugs are administered. States are using the replacement drug because the sole U.S. manufacturer stopped making it.