The problem is that cannabis is a complex mixture of natural compounds, none of which are patentable.
The same is true of tobacco actually, with many nicotinic compounds that might well have some medical efficacy. But so far companies have struck out trying to isolate and make drugs in that area.
Western medicine is focused on finding drugs that do one thing well. Many natural compounds potentially do good things across multiple areas, but the effect size is almost always small. So trials to prove efficacy are very hard, and the lack of patents makes things that much harder.
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