I personally think it's a bad idea if it's allowed in public.
If they use it recreationally indoors, in private, away from children or people not interested in second hand smoke, then it's fine.
Marijuana smoke is about as carcinogenic as sitting next to a wood fire. Not all smoke causes cancer, they've only found that specifically with tobacco derivatives that contain nicotine.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1277837/Available scientific data, that examines the carcinogenic properties of inhaling smoke and its biological consequences, suggests reasons why tobacco smoke, but not cannabis smoke, may result in lung cancer.
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Recently, Hashibe et al carried out an epidemiological analysis of marijuana smoking and cancer. A connection between marijuana smoking and lung or colorectal cancer was not observed.
Blends however (tobacco + marijuana) will have rates of harm equivalent to the amount of tobacco included, and since it's hard to police that, then no-smoking zones would also need to include joints. But purely because you can't determine what's in the joint - whether it's pure cannibis (no evidence of harm from second hand smoke, and no cases of emphysema in users) or tobacco (causes all sorts of health problems and hurts bystanders).
Honestly, I'm against legalizing it. We already have two major recreational drugs and we definitely don't need a third. Yes, smoking might be more dangerous, but there are still significant effects from smoking marijuana (9% addiction rate, eventual memory loss/damage, general issues associated with long-term stimulant use). Don't forget about lung and related damaged from smoking it (because some people will do that anyway, despite other options) and judgement impairment issues. As for hypocrisy in leaving alcohol/tobacco legal, I'm also for illegalizing tobacco (it has truly gigantic costs to everyone) and for at least limiting alcohol consumption (due to lesser cost and some religious use).
1) We already have pot here. "don't need a new drug" is head in the sand logic, because it's already here and extremely popular. It's here and never going away, which is the flaw in the "we don't need no new drug" argument.
http://www.theguardian.com/science/blog/2014/jan/17/cannabis-memory-loss-cbdAnother important component, CBD (cannabidiol, which works by increasing natural cannabinoid levels in the brain) is associated with the calming, anti-anxiety effects of the drug. In addition, CBD is thought to protect against many of the potential negative effects of marijuana, including dependence, psychotic symptoms and cognitive impairments.
The THC concentration in cannabis has increased by as much as 12% over the past 30 years, making the drug much stronger than it used to be. At the same time, there has been a significant depletion of CBD, sometimes to levels as low as 0.1%. "Skunk", as this new strain of high-THC/low-CBD marijuana is called, is flooding the illegal marijuana market, and it is this variety that is thought to be behind the rise in cannabis dependence diagnoses, links to schizophrenia, and cognitive deficits seen over the past decade.
The changing chemical make-up of cannabis appears to be partly accidental and partly deliberate. New strains are often bred to have higher levels of THC in them, increasing the drug's potency. However, modern growing techniques have also affected these chemical levels. For example, illegal growers have turned to indoor marijuana farms to avoid detection. Growing cannabis locally in such farms also circumvents the need to import the drug, and guarantees a more reliable harvest. However, the 24-hour lighting used in these farms inadvertently reduces CBD levels in the plant. Thus, these new strains are not only bred for higher potency, with elevated THC content, they are also lacking the protection provided by CBD against the drug's negative effects.
The recent legalisation of recreational and medicinal marijuana in parts of the US has the potential to reduce significantly the harms caused through incarceration or criminal records for minor drug-related offences. However, it also provides an opportunity to reduce the cognitive and psychiatric harms linked to cannabis use. With this shift in drug policy, it is now possible for states to monitor the commercial production of cannabis, regulating the levels of THC and CBD present in the drug. To facilitate this, they could force growers to use strains with higher levels of CBD, and revert to more old-fashioned farming methods that don't use round-the-clock lighting.
Yeah, so many of the problems are strongly associated with it being illegal. But it's not just cannibis, virtually
any product would be a lot more harmful if it's not legal and regulated. Think about backyard abortions. Very dangerous. And the same argument as above could be made against legalizing abortions in that situation: why would you want to legalize something so dangerous, thus legitimizing and spreading the harm? This circular argument ignores the strong evidence that both illegal abortion and illegal cannibis are much more dangerous than necessary as a result of them being illegal in the first place.
If pot growing is legalized, then immediately all the hydroponic indoors growing becomes too expensive, and growers switch to natural light, which is proven to reduce the addictiveness and negative cognitive effects, while adding more calming anti-psychotic, anti-anxiety effect.