Keith Humphreys examines it:
Some people believe the ‘gateway effect’ exists because early drug use primes the human brain for more drug-seeking, others argue that the friends you make using drugs as a youth are a ready source for other drugs later, and still others argue that there are factors, like impulsivity, that causes both early and later drug use. Which camp is correct? Probably, all of them.
He later adds:
Science is thankfully more accepting of complexity than ideology, which is particularly important when trying to solve a puzzle as intricate as human development. And in any case it’s rather ridiculous that discussions about drug gateway effects are held captive to legalization debates when the key gateway drugs (alcohol and tobacco) are already legal.