What comes as little surprise to those familiar with the ECS (endocannabinoid system), a recent Harvard medical study showed that smoking cannabis over a three month period improved cognitive performance. What???
So, since cannabis contains cannabinoid compounds that work in identical fashion to the same endocannabinoids produced by your body, it would make sense to have cognitive improvement, all other things being equal.
However, there is a major case to be made for the 'entourage effect' made famous by Israeli scientist Dr Mechoulam in 1998. There are two main chemicals that make up approximately 98% of the cannabis plant, THC and CBD. However, there are also approximately 90 other cannabinoid like compounds that act synergisticly with the other cannabinoids to produce varying effects. When the cannabinoids are used synergisticly, we get an 'entourage effect' of more effective medication.