Give me 100 random people, a year, the right environment, and some strong opiates, and I'll give you back 95 addicts.
While some might be more susceptible to drug addiction than others, getting addicted and forming dependence is nothing more than a process.
In general (outside the extremes of the dataset) your life's circumstances and details are more important to developing (and maintaining, and quitting) your drug habit than your brain chemistry is.