Which of the following is a common adverse reaction to phenobarbital?

• A. CNS depression
• B. Weight gain
• C. Insomnia
• D. Hypertension

Answer: (A)

Phenobarbital works by enhancing GABA’s inhibitory effect in the brain, which causes CNS depression. At therapeutic doses, this manifests mainly as sedation and drowsiness, making CNS depression the most common adverse reaction you’d expect with this drug. The other options don’t fit as well: weight gain isn’t a typical or defining effect of phenobarbital, insomnia would run counter to its sedating action, and hypertension is not a common adverse reaction associated with this medication. You might also see dizziness or ataxia and, with higher doses or in combination with other depressants, greater CNS and respiratory depression.