Marijuana, at least in my opinion, should be completely legalized. Many, many studies have shown that it has no negative effects whatsoever.
Completely wrong right off the bat. Even if you ignore anything that suggests it's a gateway drug, or the theory that it can trigger schizophrenia in people with a genetic predisposition, the mere fact you're burning something, inhaling it and holding it in your lungs for a long period of time is harmful.
You're also ignoring the societal issues which, while obviously not as bad as harder drugs like heroin or even arguably a legal drug like alcohol, are still something that has to be considered when you say something like 'has no negative effects'
Edit:
Personally, I think the decriminalization of medical marijuana is a total no-brainer (we have much worse and more dangerous drugs available by far).
I agree with this 100%. Marijuana has shown very positive effects when used medically, especially in people with serious or terminal illnesses or chronic pain.