"[…] the way drugs are often portrayed as nothing but harmful and damaging is quite disconcerting; it suggests that taking drugs is akin to trying to increase the speed of your computer by pouring coffee over the motherboard; you’re going to experience a lot of new sights, sounds and smells, but cause irreversible damage in the process. That’s not how it works. Many drugs are effective because they work on systems in the brain that are already there. Opiates like heroin work on opiate receptors, cocaine affects the dopamine system (amongst others). The brain and body have evolved over millions of years to recognise and utilise these chemicals, and drugs typically work because they are analogous to the substances that occur naturally in our bodies. E.g. cannabis works because the brain has endogenous cannabinoids."
Dean Burnett on the recent study that states that cannabis use is linked to reduced IQ.