Kevin Sabet directs us to studies reconfirming the negative impact of cannabis on IQ in teenagers.

A study published last year found:

Around 1000 people all born in the same year in the New Zealand city of Dunedin were interviewed at 18, 21, 26, 32 and 38 about their cannabis use. The participants were also tested for their cognitive abilities at age 13 before starting to use cannabis, and at age 38.

The study found persistent cannabis use during teenage years was associated with a drop in IQ of eight points by the age of 38.

Critics complained that the study mistook cause for effect: that it was low socioeconomic status that caused heavy pot smoking, not heavy pot-smoking that depressed life outcomes.

However,

a further analysis of the original data found no evidence for Rogeberg's hypothesis and that the same drop in IQ occurred when only middle-class cannabis users were analysed.

"By restricting our analysis to only include children from middle-class homes, our findings of IQ decline in adolescent-onset cannabis users remain unaltered, thereby suggesting the decline in IQ cannot be attributed to socio-economic factors alone," says [Madeleine Meyer, author of the original study].

Multiple studies find that heavy and persistent cannabis use negatively affects mental health, relationships, life satisfaction, education and employment.

Or, in the words of a coauthor of the Dunedin study,

"There are a whole stack of things that are correlated with this heavy pattern of use and it's unlikely that they're all artefacts of something else that is unconnected to cannabis use."