To treat mental illness effectively with pharmacological agents, a drug must be able to cross the blood–brain barrier (BBB) at sufficient amounts to provide therapeutic concentrations within brain tissue at a desired target. The challenge of drug delivery to the central nervous system (CNS) has been a longstanding problem, which has resulted in more than 98% of CNS drugs failing to enter the clinical setting because of poor BBB penetration1. This article discusses first how the BBB generally limits drug delivery to the CNS; secondly, the role the BBB plays, if any, in limiting antidepressants access to the CNS; and, finally, general strategies to bypass the BBB in new drug development for mental illness.

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