Because of the way a drug is affected by our DNA and metabolism,  there can be adverse drug reactions when one drug creates a problem with the proper metabolism of another drug.  Most of the drugs we take can act as serious toxins in the body if they are not processed properly by the body–some are serious toxins even if metabolized properly.  Many times a drug will not allow another drug to be properly metabolized and create annoying or serious side effects in the body.
Adverse drug reactions are reactions that occur when a person is given a drug which causes a serious medical problem.  According to the CDER:
• There are over 2 million serious adverse drug reactions each year.
• Adverse drug reactions cost over $136 BILLION yearly.
• $136 BILLION is greater than the total costs of cardiovascular or diabetic care.
• These statistics do not include the number of adverse drug reactions that occur in ambulatory settings (where patients are not in a hospital overnight).
• It is estimated that over 350,000 adverse drug reactions occur in U.S. nursing homes each year.
• The exact number of adverse drug reactions is not certain.
• Adverse drug reactions cause one out of five injuries or deaths per year in hospitalized patients.
• Adverse drug reactions cause over 100,000 deaths annually.
• Only heart disease, cancer and strokes are more fatal to Americans each year than adverse drug reactions.
• Adverse drug reactions increase dramatically after a patient is on four or more medications.
• Adverse drug reactions are, for the most part, preventable.  (Emphasis added)

Of course, some adverse drug reactions may occur when someone takes more of a drug to which they are addicted, but most are from the use of prescription drugs that are taken as prescribed.  It is no surprise to us that adverse drug reactions are, for the most part, preventable.  Taking a drug that is metabolized entirely or in part by the same enzymes as another drug can create problems.
One of the primary reasons for the escalating number of adverse drug reactions is that more and more people are taking more than one medication at the same time.  Here are the facts:

• 64 percent of all patient visits to physicians result in a prescription (often determined by the doctor in a visit lasting only seven minutes).
• Of all the prescriptions written by doctors, 2.8 billion prescriptions were actually filled in the year 2000 in the United States–an average of about ten prescriptions for each individual in the United States.
• Certain antidepressant, antipsychotic and opiate drugs inhibit the CYP2D6 enzyme to the point that as many as 80 percent of the people who were extensive (normal) metabolizers become poor metabolizers of drugs metabolized by the CYP2D6 enzyme.

If the metabolism of a drug is inhibited, or inhibits or speeds up the metabolism of other drugs, the resulting buildup of either or both drugs:
• Can cause serious side effects.
• Can cause a drug, like blood pressure medicine, to not be metabolized and not provide the needed benefit—like control of blood pressure.
• Is likely to cause a severe adverse reaction.