Conventional Units ( ug /ml) SI units ( uM /L) Therapeutic Range, Epilepsy 15 - 40 ug /mL 64 - 172 uM /L Mild signs of toxicity usually noted (e.g. ataxia, nystagmus) >50 ug /mL >215 uM /L Severe toxicity can occur (e.g. stupor/coma) >65 ug /mL >280 uM /L Therapeutic range, Monotherapy for EtOH withdrawal ~10 - 40 ug /mL (??) ~43 - 172 uM /L (??) interpretation of phenobarbital levels This table is only intended to provide a rough concept of phenobarbital levels. The optimal phenobarbital levels in treatment of alcohol withdrawal remains unclear. Ultimately, doses need to be titrated based on clinical response. For example, patients with alcohol withdrawal will often have an excellent clinical response at phenobarbital levels which are below the traditional therapeutic range for epilepsy. - The Internet Book of Critical Care, by @PulmCri t