No, Virginia, cannabis does not cure cancer

As Canada lurches towards marijuana legalization next year, the scores of pot dispensaries that have proliferated (mainly illegally) are using all sorts of iffy advertising to move their product. A common trope is that cannabis can somehow mysteriously help or even cure cancer, and that at the very least it reduces chemotherapy-related nausea. Claims like "significantly reduces the ability of the cancer to spread" are bandied about, and anecdotal case studies of supposedly miraculous remissions are presented as scientific evidence.
Now, anecdotes are not evidence, however much you might like them to be. Without rigorous medical documentation, there is no way to tell whether it was the cannabis that helped, or previous radiation and chemotherapy treatments, or some other entirely unknown factor, or whether it was just a spontaneous remission, which also happens. So, what, then, is the actual scientific evidence behind these claims. Unfortunately, there really isn't any. Or, at least, not yet.
There have been some scientific studies done, and many more are under way or in the planning stages. Some have indicated that there may be some positive effects, perhaps enough to justify further studies and tests, but far from conclusive or definitive evidence. Others studies, however, have indicated that cannabis may interfere with the immune system's tumour-supressing role and could even stimulate cancer cell growth. Also, the high doses of cannabis-based THC or CBD oils that are often recommended for cancer treatments may also decrease quality of life, leaving patients psychotic or dangerously sedated.
The conclusions of one major meta-study may be the best summary of the situation: "there is not enough evidence for any of the cancers to state confidently that it is effective and safe". Perhaps, the best that can be said for cannabis is that it may reduce reliance on high-dose opioids, which can have even more negative side-effects. As such, it could be used as part of an overall risk-reduction strategy.
The good news is that many more studies are currently under way, and some teams seem very confident that a breakthrough may be imminent (read: within 10 years). Time, though, and not wishful thinking, will tell.