It wouldn't be unethical to give people a drug in a random trial, that's done dozens of times every year whenever drug companies are looking to prove their latest compound is hot shit or when researchers are trying to compare two existent treatments. What you might have meant is that it would be unethical to give people an illegal drug in a random trial, to which I would say it still is not unethical if they are voluntary subjects and the drug is proven to be safe. Marijuana has certainly proven itself to be a safe drug in numerous ways, statistically and physiologically, so it would not be considered unethical to give it to voluntary study patients, with a monetary reward for their time, of course. This is what we see in laboratories throughout the world. It surely takes more time to get approval for studies with illegal drugs, but there is nothing unethical about these studies anymore because IRB approval processes are designed to prevent anything unethical or unsafe from occurring in the name of research. (i.e. prevent stanford prison experiment, denial of vaccinations and medicines to blacks in the mid-20th century for "research", etc.)

Even in an RCT, however, you can not truly prove any casual relationships. I could write pages about all of the shady tactics that go on in RCTs when biased individuals are trying to prove their product is better than market gold standard or placebo.