Pharmacogenetics - personalized psych-drugs

Pharmacogenetics is the study of how genetic idiosyncrasies influence responses to drugs of each person. In 1995 Michael Adams-Conroy, a nine years old boy, died due to Prozac overdose. His parents were accused of giving him a premeditate overdose but it was discovered that the boy had a "CYP 2D6 gene defect" - lacked the enzyme that metabolizes the drug.

I've just discovered this site for DNA Drug Reaction Testing.

"Research shows that of all the clinical factors such as age, sex, weight, general health and liver function that alter a patient's response to drugs, genetic factors are the most important. This information becomes even more crucial when you consider the fact that adverse reactions to prescription drugs are killing about 106,000 Americans each year- roughly three times as many as are killed by automobiles. This makes prescription drugs the fourth leading killer in the U.S., after heart disease, cancer, and stroke." (emphasis mine)

I'm confused. So, adverse effects are due to genetics? Why are we having same side effects and withdrawal symptoms if...
I'm really confused especially after finding this article "The 5 Myths of Pharmacogenomics" which assures the Pharmas that they will not reduce their profits.
I'll be glad if anyone can help me understand it all.