"Do you have to have a low IQ to smoke cannabis in the first place?"

No! You just have to be a conformist and bow to peer group pressure. ;-) I know this, having seen it all around me in my own youth. Being a lifelong nonconformist, I didn't join in, but virtually everyone I knew did, including some very smart people. Virtually everyone smoked back then too. I didn't. Virtually everyone drank. I did that very moderately, not enough to cause me any harm. (In short: if all the world's alcohol had disappeared, I would hardly have noticed.)

"Young pot smokers run risk of lower IQ"

So do young drinkers and smokers, I've noticed. ;-) Why does this surprise anyone? Does this mean that pot, alcohol, and cigarettes should all be made illegal...along with everything else that causes risk of a lower IQ? If so, how would we effectively enforce such laws? (Answer: we would not be able to...and the laws would create a larger problem than the original problem they were put in place to address! Historical evidence for this: Prohibition of alcohol...it was an unenforcable disaster.)

You cannot FORCE a general public to make all of the best personal lifestyle choices that you wish they would by intimidating them with punitive laws and treating them as criminals. If you try to do it, you will fail, because people like deciding for themselves how they are going to live and whether or not they are going to eat healthy food, injest healthful substances, exercise, etc.

What you can do is regulate the marketing of food and other common substances with certain basic standards of safety...as we do with food that is bought at grocery stores, for example. But you can't stop people from eating using such substances altogether...or at least you shouldn't, because it should be their choice to eat pork if they want to, smoke tobacco if they want to, have a drink if they want to, and smoke pot if they want to. It's their business, and nobody else's.

You also can't FORCE other people to be smart. Or to be good. Or to be kind. But you CAN set a good example yourself. That would be more effective, seems to me.