What is the effect of edema on distribution into the brain?

• A. Edema decreases interstitial fluid, limiting distribution
• B. Edema increases interstitial fluid and may alter blood-brain barrier function, potentially affecting CNS distribution
• C. Edema has no effect on brain distribution
• D. Edema always increases brain penetration for all drugs

Answer: (B)

Edema in the brain expands the extracellular/interstitial fluid space and can disrupt the blood–brain barrier. The increased interstitial volume changes how drugs diffuse through brain tissue, often diluting concentration or altering diffusion paths. If the barrier is compromised, permeability to many compounds rises, allowing greater brain penetration. Because edema can both expand distribution space and modify barrier function, its effect on CNS drug distribution is not fixed and depends on the drug’s properties. That combination—more interstitial fluid and potential BBB alteration, with the result being variable CNS distribution—is why this option best captures the effect.