Phenobarbital therapeutic plasma level is within which range?

• A. 4-12 mcg/mL
• B. 10-40 mcg/mL
• C. 50-100 mcg/mL
• D. 0-10 mcg/mL

Answer: (B)

The main idea is understanding the therapeutic plasma range used to guide phenobarbital dosing so you achieve seizure control without causing toxicity. For phenobarbital, the commonly targeted trough concentration is about 10 to 40 mcg/mL. This window tends to provide enough drug to prevent seizures while keeping the risk of significant side effects low. Levels below 10 mcg/mL are usually not enough to control seizures, while levels above 40 mcg/mL raise the likelihood of marked sedation, respiratory depression, and other toxic effects. Individual factors like age, liver function, drug interactions, and dosing schedule can shift this range, so clinicians monitor trough levels and adjust as needed. Among the options, 10–40 mcg/mL is the appropriate therapeutic range.