There's a lot of conflicting information regarding this question and people have different agendas. I'm not a doctor, but from what I've read marijuana has been shown to inhibit healthy brain development in adolescents and lead to lower IQ scores. And although it's not nearly as addictive as opiates, like all other psychoactive drugs that work on the dopamine system there is the potential for psychological dependence.
However, my question relates to adults that use it responsibly. If one is over 25 (brain stops developing), active, and only uses marijuana occasionally without smoking is it unhealthy? Does it cause cognitive decline in that population? I think smoking all day, every day is a horrible way to live. Those people have long forgotten what it means to "feel their emotions." However, I'm still not sure whether it is considered unhealthy when used sensibly like the occasional glass of wine.