I'd urge any of you to bear in mind a couple of things that I believe research has shown (or not) about this whole thing.
There are differences between the brains of substance abusers and those who don't use. However, it is not yet know whether these differences pre-date the substance abuse i.e. it may be the substances themselves that have caused the changes.
Also, it has not yet been proven that there is an addiction gene. There was a huge fanfare about it a few years ago, however the subsequent doubts regarding the study by the same research team didn't get nearly as much publicity. The fact that there is addiction within a family does not prove a genetic link rather than an environmental one.