Phenobarbital is a barbiturate, well actually it is the one and only barbiturate that remains being used medically.
It has a slow action.
The brand mostly known is Luminal, at least in Europe.
DWD Oregon recommended its use amongs their associates instead Pentobarbital, because the high price of pento. Of course, doing a combo with a benzo and a sedative.
Pheno has a lethal dose of 6 grms.