EDIT: This post ended up a lot longer than I intended. Appy polly loggies.
I don't see why we should legalize a drug just because more harmful substances are available. Its like saying we should allow items in brawl tournaments just because there is already randomness involved. Thats just making things worse.
Except marijuana isn't really very bad for you. Let me go through the negative effects:
Tar in the lungs - negligible by vapouriser use or cooking.
Cancer from combustion of plant matter - negligible by vapouriser use or cooking.
All non brain related health concerns - negligible by vapouriser use or cooking.
Vapourising is easy. I have a vapouriser I made out of a light bulb that works plenty good and took about 5 minutes to make.
I would like to say before anyone says 'very few people vapourise marijuana' that this is partially due to lack of knowledge of the existence of vapourisation and also lack of knowledge of its benefits. People could easily be educated about the benefits of vapourisation were marijuana to ever become legal. And it still remains that many of the effects of marijuana only exist if you choose for them to exist. Smoking is bad for you whether you smoke lights or not, and drinking will always be bad for you too. It's optional when taking marijuana to have it increase cancer risk (though SOME studies have shown that cancer rates in marijuana smokers are comparable to those of non-smokers.
link)
Now for the brain.
Links to schizophrenia. To analyse this we first have to know why schizophrenia is caused. While there are genetic reasons, prenatal reasons and social reasons (your environment, social situation), the reason marijuana can lead to schizophrenia is essentially by over-stimulating dopamine receptors in the brain over a long period of time. THC happens to release a lot of dopamine in the brain. So surely THC will increase risk of schizophrenia? Yes. THC will, but it is not the only cannabinoid in marijuana. This sounds like a cop out at first, until you learn that the second most occurring cannabinoid is CBD (up to 40 of the cannabinoids in marijuana in some indica strains). This is important, as CBD is an anti-psychotic, as potent as many of the drugs we use to combat schizophrenia (
link to study). The presence of CBD in marijuana negates the risks of schizophrenia that THC causes.
So why do we see studies linking marijuana to schizophrenia? Because of the polarisation of cannabis strains over the past decades. CBD limits the effects of THC somewhat and makes the high more sedative, and so breeders set about to create strains with very low amounts of CBD and high amounts of THC to give an energetic and euphoric high. This high gained the polarised strains popularity (especially Skunk and its variants here in the UK, which have very very little CBD), and so without CBD to keep it in check, the high THC levels in the marijuana began to give a small percentage of heavy users (around 2% of heavy users) schizophrenia.
This is incredibly relevant to legalisation, as with government standards in place, such polarised strains could be either not grown, or their health risks could be taught to users. Pure indica strains such as Kush have such high levels of CBD that they could theoretically be useful in actually treating schizophrenia and depression. And on a side note, CBD has also been shown to relieve nausea, and
inhibit cancerous tumour growth. Combined with a vaporiser, weed could be GOOD for you, not just a drug with little negative effect.
I will not refute that over use of the drug can have disastrous consequences. Especially in students, as while marijuana does not kill brain cells or anything of that sort, the cannabinoids that enter the blood stream from smoking are partially absorbed in to the fatty tissue in the brain. When under stress, this fat releases the stored cannabinoids, making learning, and specifically exams, difficult for users.
Legalising WOULD make marijuana harder to obtain for those who are below the age needed to buy it. Weed is notoriously easy to get ahold, and even as a minor I have much easier access to it than alcohol (a regulated drug). I smoke a lot less than I drink, too. This would cut down on marijuana use significantly in youth, making it a positive step to legalise for students, not a negative one.
Of course abuse of the drug will have bad consequences. But nothing's stopping people from abusing it now and the war on drugs has proven miserably ineffective. Not to mention, the lack of an addictive compound in marijuana reduces the likelihood of people becoming addicted. Should we ban rope because some people choose to abuse it and hang themselves with it, or use it to tie up my sister and beat her when we were kids but we're past that now? No. That would be crazy.
Granted this particular drug may not be that bad, but I still haven't seen a strong reason for non medical marijuana to be legalized.
And I haven't seen a particularly good reason to keep it illegal. Do we really need to go over all the benefits of legalisation AGAIN? I'm sure they've been mentioned in this thread. Some people like to smoke weed, it isn't particularly bad for them, so why attempt (poorly) to stop them from doing it? Quick list of benefits of legalisation to refresh your memory: economic from taxing sales, from freedom to use industrial hemp and from stopping spending millions to facilitate peaceful offenders in jails; social for cutting down use in teens, granting extra freedoms. I've probably left some out, so anyone else can add if they want.
EDIT: To whomever said that marijuana is a gateway drug. This is similar to saying we should keep cannabis illegal because of the drug trade built up around it. There are two reasons this 'gateway' effect exists. Firstly, at the moment buying cannabis brings users in direct contact with drug dealers, some of whom may sell other drugs and may push these drugs on the cannabis user. Secondly, the status of cannabis as illegal associates it with other illegal drugs, hence the mentality that anyone willing to smoke cannabis is also willing to try other illegal drugs. Without the stigmatism cannabis has developed as an illegal drug, saying it is a gateway drug would be similar to saying that someone who has tried drinking alcohol would try illegal drugs.