Medical research shows two major causes of physical addiction. First, your cells adapt to the drug and, second, your metabolism becomes more efficient. To your cells, the drugs you're using become a way of life. Every time you use a drug, your blood carries it to every cell in your body. Your cells adjust. They grow to expect these doses on schedule. Your cells learn to cope with various drugs by defending themselves against the drugs' toxic effects. Cell walls harden to retain stability and reduce toxic damage. But as your cells get tough against drugs, gradually more and more can be consumed. Your tolerance increases. In the long run, however, cell walls break down. At this point, your cells not only lose their ability to keep toxins out but also become unable to retain essential nutrients. Many of them stop functioning altogether or start functioning abnormally. That's when your organs (heart, brain, liver, or lungs), which are nothing more than whole systems of cells, begin to fail.

The problem with metabolism is that it is intimately connected to diet. Your body metabolizes food (breaks it down into its constituent parts) to get vital nutrients to all the cells. To serve this purpose, your body can metabolize many different foods and can learn how to gain nutrients from almost any kind of food you give it.

Metabolism also helps to rid the body of unwanted toxins. The liver is the key organ in this process. The liver "sees" drugs as unwanted toxins and begins producing enzymes that will help eliminate them from the body. It produces a different combination of enzymes for each drug. Moreover, the liver becomes extremely efficient at producing these enzymes. The more it "sees" a particular drug, the more efficiently it produces the enzymes that inactivate that drug.Thus, a drug that you use often will get eliminated from the body with greater and greater efficiency. It's as if the liver begins to "expect" that drug and has enzymes ready and waiting. This is a key reason that tolerance increases, that is, why it takes greater and greater doses of a drug to get the same original effects.

Yet your personal metabolism works differently from anyone else's. Studies show that each individual has a unique biochemical makeup and that individuals differ greatly from one another in the way they metabolize different foods, drugs, or toxins. To give you an idea how much possible variation there is, researchers have presently identified over 3,000 metabolic substances (called "metabolites") and over 1,100 enzymes. Each individual has different proportions of all 4,100 of these bio-chemicals. Of the enzymes, only about 30 are responsible for metabolizing all drugs.