Which statement correctly describes the blood-brain barrier's effect on drug action?

• A. It prevents protein-bound and highly ionized drugs from crossing into the CNS, but does not slow the effects of drugs that can cross.
• B. It blocks all CNS-acting drugs.
• C. It speeds the onset of CNS drugs.
• D. It has no influence on CNS drug distribution.

Answer: (A)

The blood-brain barrier shapes CNS drug effects by controlling which substances can reach the brain. Only the free, nonionized, and lipophilic portion of a drug tends to diffuse across the barrier, while most protein-bound and highly ionized drugs are largely prevented from entering. That means high plasma protein binding or strong ionization keeps drugs out of the CNS, producing minimal CNS effects. For drugs that are unbound and lipophilic, crossing the barrier occurs more readily, and once inside the CNS, the barrier no longer slows their action; the onset and intensity depend on how quickly the drug reaches and interacts with CNS receptors. The barrier does not block all CNS-acting drugs, and it influences distribution by limiting CNS exposure to many drugs that cannot cross.