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When heroin, or morphine are eaten, smoked or injected, they are
ultimately carried to the brain by the blood. In the brain, they attach to
certain cells, in certain areas of the brain, that have specific structures,
now named opiate receptors. These brain cells, called neurons, then
release a chemical messenger or "neurotransmitter" called dopamine,
which, in large amounts, can cause the rapturous experience known as
the "high".

But there are opiate receptors in other organs in the body. For
example, in the intestines, when opiates attach to the receptors, they
slow down the intestine, which is why opiates have been used as
treatment for diarrhea. In the lung there are sensors that detect the
presence of minute particles that can cause damage to the delicate
tissues. These sensors send signals to certain parts of the brain that
initiate the cough reflex that expel the particles. Opiate receptors
decrease the sensitivity of the neural network that leads to coughing,
which is why opiates are used in upper respiratory infections to
suppress cough. (Even though methadone does not have a plant origin,
it attaches itself to the same opiate receptors.)

All organisms are in a constant state of renewal. Cells and tissues are
constantly being removed and replaced. Small molecules, which are
manufactured by the organism, are also chemically altered and
excreted. The same is true for medicines, which is why one has to
take many medicines on a regular schedule so the desired benefit won't
wear off.

Heroin is removed from the body in a few hours. It is chemically
altered by the liver to make it water soluble, and then is excreted by
the kidneys in the urine. As a consequence, heroin addicts must "fix"
three or four times a day. As was mentioned above, methadone is
removed much more slowly, so that a single daily dose is usually
sufficient to maintain a therapeutic level.

One of the most important functions of the brain is to maintain balance
of all the chemical reactions that ultimately define life. Many of the
reactions that are constantly taking place twenty-four hours a day,
throughout our lives, are controlled by facilitators called enzymes,