"Cannabis is correlated with schizophrenia."
This line of argument is an insidious attempt to convince the ignorant that cannabis causes schizophrenia. In fact, there is no scientific evidence to support this. Correlation does not equal causation, but this distinction is too subtle for the average person who reads the above headline.
There is evidence to suggest that those who are prone to schizophrenia will more likely use cannabis, likely because the cannabis eases the negative symptoms of the schizophrenia.
The rate of cannabis usage has gone up tremendously in the last forty years. If cannabis were truly a cause of schizophrenia, we would have seen schizophrenia cases increase at a proportional rate. What we see instead is that rates of schizophrenia are steady for this period. Therefore, there is no causal link between cannabis use and schizophrenia.
What prohibitionists who use the above argument will never do is to compare that correlation to that of alcohol and all manner of mental health problems. No, they always insist on looking at cannabis in isolation, never mentioning that alcohol has a much clearer causal link to mental disorders than cannabis ever could. The same is true for countless other legal substances and risky behaviors.
Finally, citing any specific harm of cannabis as a reason to keep it illegal is just specious. If it is truly harmful, then there is all the more reason to bring it under the control of a legal and regulated system. Leaving its trade to organized criminal cartels allows it to cause so much more harm than it otherwise would. Countries that have enacted policies of Harm Reduction over prohibition have seen their rates of usage decrease, even as harms associated with drug use and the illegal drug trade have all but disappeared.