Final answer:
The sixth barrier drugs encounter en route to their target organs is often the blood-brain barrier (BBB), which prevents many substances in the bloodstream from entering the CNS.
Step-by-step explanation:
The sixth barrier a drug must pass to reach its target organ is commonly the blood-brain barrier (BBB). This barrier is essential in protecting the central nervous system (CNS) by preventing many substances circulating in the bloodstream from crossing into the CNS. Nutrient molecules can pass through the BBB, but most drugs cannot without specific modifications to their structure. Pharmaceutical companies must design drugs that are able to cross the BBB to reach their targets within the CNS. This design often involves altering the drug's physicochemical properties, such as lipophilicity, to enable it to penetrate this formidable barrier.