In cases like this the onus is always on the person that's arguing that it's safe. Basically because the harm of not doing is far less than the possible harm of doing in most cases.
Your posting history seems to indicate that you are a reasonable and intelligent person, so I have to assume that you mean something very specific by "cases like this". Because if "cases like this" is interpreted as even a somewhat general thing then you seem to be saying that by default any (physiologically altering?) substance should be illegal until it has been proven to be safe for several decades.
I think rather the opposite: that a substance should only be made illegal if it is proven to be harmful--and what is more, it should only be made illegal if it directly harms a person other than a person consenting to be harmed by it.
I'd like clarification on what cases you think should be illegal by default. Maybe you could suggest a blanket law that would define a class of things as illegal for intake until they are approved?
Disclaimer: I have never tried marijuana or any other illegal drug.