So we don’t have clear evidence that smoking kills; it could be that most or all of the death-smoking correlations are due to selection effects, and not smoking causing death. ... Apparently we need bigger trials if we are going to see clearly if smoking kills. Alas the era of the large risk trial seems to be over, at least for now; it seems it will be a long time before we really know.
Marijuana does not cause people to use hard drugs. What the gateway theory presents as a causal explanation is a statistic association between common and uncommon drugs, an association that changes over time as different drugs increase and decrease in prevalence. ... Indeed, for the large majority of people, marijuana is a terminus rather than a gateway drug.