Phenobarbital is a ln antiepileptic drug which is mainly used to prevent seizures in people suffering from epilepsy. Phenobarbital test is used to measure or monitor the phenobarbital level in the blood and in turn, determines whether they are within a therapeutic range or not. After the results, a health practitioner decides on the dose of the drugs. This test is mainly done to make sure that the phenobarbital level in the blood is optimal. High as well as low phenobarbital level is equally harmful as it can have various adverse impacts on the body.