There is some evidence that there might be a genetic component to some drug misuse disorders. Specifically, I know that there's been some research done with nicotine here (either out of Columbia or Radboud, don't really remember). Most researchers are still of the opinion that environmental factors have much more effect on drug misuse disorders than genetic.

The trope of "your first dose will get you hooked" is patently false. There's no evidence that this is true. Anecdotal anything, especially with drugs, doesn't mean anything, and all of these outlier anecdotes are actively damaging to the education of the public with regards to drugs.