Consider more transient alternatives: there is some evidence that other medical routes are open for addiction, but in most places the experiments needed to validate them are illegal.
For example, there have long been anecdotes about classical psychedelics (and some things less-well-understood) acting as treatments for addictions.
This includes recent claims from John Hopkins researchers that up to 80% of smokers enrolled in a psilocybin study ceased smoking for clinically significant lengths of time after the study: however, this was only an observation and a dedicated study hasn't yet been done AFAIK.
Yet, it is possible that the anti-addiction effects are inextricable from the psychedelic effect, which is making it extremely difficult to study for legal reasons. There are signs that this may change, as a few cities have begun decriminalising psilocybin recently, and it may go the same way as cannabis and become a steady movement towards legalisation. But look how long that's taking for cannabis!
Anyways: not all things that show antiaddictive value are illegal, in all places. Apparently there is a body of research surrounding kappa opioid receptor agonists like Salvia divinorum, though I think the safety profile there is far less well-established than for classical psychedelics and research is very slim. I gather that's not illegal in some parts of the US and EU.
Genetic engineering is great, but it's probably not the best route for conditions like addiction, IMO. There is so much research to be done on neurotransmitter modulation first, and lots of lost time to make up since that research largely got banned decades ago.
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