is not an actual fallacy, unless your claim is that smoking marijuana automatically leads to using harder drugs. Based on my ample observations in high school where some of my class mates became drug addicts, they all started with marijuana. Nobody who didn't smoke marijuana started on opiates. And I observed no marijuana smokers who hadn't started with tobacco. The non-smokers never did.Gateway fallacy
So I am not saying that it is impossible to get into drugs without the 'gateway', but in my experience, for the majority of people who ended up becoming drug addicts, the road from tobacco --> marijuana --> opiates seemed to be very common even if many marijuana smokers never moved on to harder drugs, so it is only a fallacy if any kind of determinism is claimed. Later, when I was working for a year with drug addicts in rehab, I only met one who didn't smoke tobacco. She was also into healthy eating - an exception in this respect, too.
There is also no determinism in the idea of yoga --> new age, and I think that more people who never practiced yoga are into new-age stuff, than drug addicts who never smoked marijuana or even tobacco.