Unfortunately, research has been prohibited until relatively recently, so no long-term studies have been possible. Not that long-term effects of anything medicinal have really been studied and aren't just generally evaded by pharmaceutical companies.
I occasionally smoke marijuana, and have done so for years. In college, I smoked every day, and even after a year or two it became unsustainable. You just slowly start falling behind in life when the general meta in humanity is consistent growth and progression.
The artificial ups and downs can become mentally and emotionally tedious. The flatline of sobriety is generally more desirable, as the body does not have to consistently struggle to maintain equilibrium.
I still like to break the circuits now and then though.