Sedation and hypnosis are the principal side effects of phenobarbital. Central nervous system effects like dizziness, nystagmus (involuntary eye movement) and ataxia
(unsteady and clumsy motion of the limbs and torso) are also common. In elderly patients, it may cause excitement and
confusion while in children, it may result in paradoxical hyperactivity.1
1. http://en.wikipedia.org/wiki/Phenobarbital