True or False: A drug affecting the brain requires crossing the blood-brain barrier.

• A. False
• B. Depends on the drug
• C. True
• D. Sometimes

Answer: (C)

The main idea is that getting into brain tissue is usually necessary for a drug to have direct central effects because the blood-brain barrier is highly selective and blocks most substances circulating in the blood. For a drug to influence brain function directly, it must cross into the brain, which generally happens if the molecule is lipophilic enough to diffuse through the membranes or can be transported by specific brain transporters. There are ways to reach the brain without traditional crossing (like nose-to-brain routes), but those still involve the drug ultimately entering brain tissue to exert its effects. So, true is best: affecting the brain typically requires crossing the blood-brain barrier.