Heroin

General 2: The blood-brain barrier

Heroin molecules can easily penetrate the blood vessel walls forming the blood-brain barrier, even though these are better protected against foreign substances than other blood vessels in the body.

Heroin passes through this barrier much more easily than morphine. That is why morphine is processed to manufacture heroin. Once in the brain, heroin quickly changes back into morphine. The morphine then exerts its psychoactive effects on the brain.

More details are shown in the animation Drugs and the Brain.