Metabolism is derived from the Greek word metaballein meaning to change.  It is used to refer to the chemical processes in living organisms that allow for functions such as:   
Circulation of the blood
• Maintenance of body temperature
• Muscle contraction and relaxation
• Digestion
• Elimination of waste 

DRUG METABOLISM

Once in the body, a drug must be metabolized for it to be used by the body.  Metabolism is the action of complex proteins called enzymes breaking down medications and drugs and allowing the body to utilize them. When a drug or medication is taken orally, it usually travels from the stomach to the liver, where most drugs are metabolized by enzymes.  If properly metabolized, the drug is then able to be used by the body. 
Here is an illustration of the enzymes in hepatic (liver) drug metabolism:
Cyp metabolism images

In that group of enzymes in the liver, an enzyme named CYP3A metabolizes the largest number of drugs, followed closely by the CYP2D6 enzyme.   While it is not the enzyme that metabolizes the most number of drugs, the CYP2D6 enzyme is responsible for the metabolism of an estimated 23 percent of all prescription drugs and many antidepressant, antipsychotic and opiate drugs.  

However, some people do not have the CYP2D6 enzyme at all, and for others, although it is present, it works improperly.  In addition, because a number of different drugs are metabolized by the CYP2D6 enzyme, even if the enzyme is present in the liver and works properly when one drug alone is taken, a second drug which is metabolized by the same enzyme can create a type of competition for the enzyme if it is taken at a time close to the time the first drug is taken.   This competition can result in one drug inhibiting the operation of the enzyme and preventing it from metabolizing the other drug.  The drug that is not metabolized often accumulates in the body, and can cause other unwanted symptoms or conditions (side effects) or adverse drug reactions.  

There are also some drugs that increase the activity of the enzyme.  For example, if someone is taking a blood pressure medication and is prescribed an antidepressant that causes the rapid metabolism of blood pressure medicine, the result will be that the blood pressure medicine can be rendered useless for lowering blood pressure.  

Unfortunately, not only drugs but some herbs, vitamins and foods can also affect enzyme activity.  For example, grapefruit juice can act as an inhibitor of CYP3A, the enzyme responsible for the metabolism of the largest number of general drugs, and St. John’s wort has been found to increase the activity of the this enzyme.

Because of genetic differences and the influences of other drugs, foods, vitamins and herbs, it is estimated that nearly fifty percent of us metabolize drugs differently, and this can lead to serious problems.

FOUR TYPES OF METABOLIZERS

People have been grouped into four types of metabolizers:   
1. Extensive Metabolizers (EM).   Individuals who metabolize a drug in a predictable and normal way.  
2. Intermediate Metabolizers (IM).  Individuals who metabolize a drug better than poor metabolizers and worse than extensive metabolizers and are more likely to have serious side effects.  
3. Poor Metabolizers (PM).  Individuals who are unable to adequately metabolize a drug, resulting in dangerous accumulations of the drug and severe side effects.  
4. Ultra Extensive Metabolizers (UM).  Individuals who metabolize a drug much faster than predicted.   This can cause too high concentrations of drugs in the body and create toxic reactions, or sometimes cause a drug to be used too quickly and its effect not last for the time expected.

Poor and intermediate metabolizers may have even more serious problems.  The drug buildup in the poor and intermediate metabolizer can be so severe that it can cause death.  

A DNA test can be conducted to determine the type of metabolizer that you are for a drug before you take it.  This should certainly be done before taking any antipsychotic, antidepressant or opiate.  The DNA test may indicate that you should not take a particular drug, or it may help you avoid adverse drug interactions and uncomfortable or painful side effects.