which strength to dispense for phenobarbital prescriptions

Questions often come up about which strength to dispense for phenobarbital prescriptions... 30 mg or 32.4 mg, 60 mg or 64.8 mg, etc.

Phenobarbital is available in "whole" strengths (15 mg, 30 mg, and 60 mg) and "decimal point" strengths (16.2 mg, 32.4 mg, and 64.8 mg).

This is because phenobarbital is an old drug that used to be dosed in GRAINS...where one grain equals about 64.8 mg.