Up to 2-6 weeks. Fluorescent dyes invalidate this assay. Phenobarbital is a metabolite of primidone and mephobarbital.
Phenobarbital affects the metabolism of phenytoin and succinimides.
It is recommended that these levels be monitored when the phenobarbital dose is changed. Phenobarbital may increase the effect of other drugs bound to protein, such as warfarin. Used with valproic acid, excretion diminishes because of urine acidification by valproic acid. In addition to legitimate therapeutic applications phenobarbital, like other barbiturates, is a drug of abuse. Ethanol and a barbiturate are a potentially lethal combination.