This test measures the level of phenobarbital in the blood to determine whether the drug level is within a therapeutic range.. Phenobarbital is a barbiturate, an antiepileptic drug (AED) and sedating drug that depresses the nervous system. Doctors usually prescribe it to prevent seizures or to relieve anxiety. It is often prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain. It is important to maintain a stable level of phenobarbital in the blood within the therapeutic range. If the level is too low, the person who is being treated may experience seizures or anxiety. If the level is too high, the individual could experience side effects or even toxicity. Sample: – – – Reference: – – – Frequency: – – –