The World Health Organization Expert Committee on Drug Dependence has come out with a rather positive outlook on cannabis. They recently released their pre-review report on cannabis and cannabis-related substances which found cannabis to have minimal adverse affects and established medical usage.
The WHO seems to be particularly confident in the non-psychoactive cannabis extract CBD, particularly in the medical space, saying, the potential therapeutic effects of CBD "[are] generally well tolerated, with a good safety profile." They cited CBD-based epilepsy medication as the premier example of this.
The whole cannabis plant got off pretty easy, too. The WHO admitted that "cannabis is not associated with acute fatal overdoses."
Additionally, they found cannabis consumption to improve sleep, appetite and able to effectively treat chronic and neuropathic pain.
As far as adverse effects of smoking cannabis, the WHO didn't come up with much, listing "euphoria, laughter and talkativeness" as potential reactions. They did note, however, that cannabis consumption can lead to anxiety and even "psychotic reactions in vulnerable individuals."
Overall though, the WHO found cannabis to be a "relatively safe drug." One that around 231,000,000 people worldwide already use.