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RocktheHouse Not even the American Addiction center fully agrees with this theory
Conclusions
There is evidence that using some substances early in development does result in a greater probability that an individual will abuse other substances; however, the reason for this is not well understood. This condition may represent some combination of inherent factors (e.g., genetic) and the interaction of environmental factors (e.g., peers, learning, stress, etc.).
The relationship between early alcohol use and later use of other drugs has the strongest evidence to suggest that it may be a gateway drug; however, the gateway theory suffers from a number of potential methodological flaws. At the current time, it is unable to specify a causal relationship between early use of any drug and the potential to use or abuse other drugs later. Instead, these relationships may be more consistent with the common liability model
https://americanaddictioncenters.org/the-addiction-cycle/gateway-drugs
And the article which you took your source from is way more nuanced as well.
