Almost all of the heavy 'serious' drug users that I see started with pot. That doesn't mean that a pot smoker will graduate to the hard stuff, but they're on the way there, even if they don't finish the journey.
And all the pot smokers started on mother's milk. Which means, using the above logic (or lack of it) that all mother's milk users are on the way to being heroin users, even if they never finish the journey.
