Another way to approach this is to look at places where legalizing, or at least decriminalizing marijuana has already been tried.
The Netherlands, particularly certain areas of Amsterdam, stopped enforcing marijuana laws decades ago, and permits a select number of "coffee shops" that are licensed and taxed to sell the drug to patrons.
The hope was that marijuana use would ultimately go down once the "forbidden fruit" aspect discussed in the answer above was removed, and then law enforcement could focus on the harder narcotics. Recent studies suggest that if anything, marijuana use has increased slightly since decriminalization, and crime related to drugs has been more localized, but not reduced. Further, it has led to a large amount of "drug tourism" where people from nearby countries without legal drugs come to Amsterdam, buy it, and transport it home at the end of their vacation.
The previous post was quite strong. I would like to suggest one other potentially challenging reason as to why legalizing marijuana could prove to be problematic. The positive vision of the legalization of marijuana depicts a world where this notion is automatically embraced. However, there will be a period of transition between the time when it has been criminalized and the time it is not. This interim will be a challenging transition, as kids would have been taught in their own experience that marijuana is wrong and then the government legalization of it would cause severe questioning of this premise. To some extent, there would be a "forbidden fruit" period, where the drug use will probably increase because it is now legal. The impact of this interim period is unknown and, frankly, overlooked. While there are distinct benefits, the short term destruction and challenge posed might overwhelm it.
This seems very similar to your question about why America SHOULD legalize marijuana and I think that's why no one answered last time. But since you ask again...
Here are what I would think are the most common arguments against legalization:
- It's a "gateway drug" -- people start by using marijuana and then they move to more dangerous drugs such as cocaine or heroin. So if we legalize marijuana we will get more people using harder drugs.
- Legalizing marijuana would send a bad message. It would imply that using drugs is okay. This would also lead to more use of hard drugs.
- It is addictive.
- Marijuana leads to various health problems, both short term and long term.
Because like other prohibited drugs, Marijuana can cause chest colds, bronchitis, emphysema, low libido, and bronchial asthma. Aside from these, Marijuana use reduces learning ability. Research has been piling up of late demonstrating clearly that marijuana limits the capacity to absorb and retain information. A 1995 study of college students discovered that the inability of heavy marijuana users to focus, sustain attention, and organize data persists for as long as 24 hours after their last use of the drug. Earlier research, comparing cognitive abilities of adult marijuana users with non-using adults, found that users fall short on memory as well as math and verbal skills. Although it has yet to be proven conclusively that heavy marijuana use can cause irreversible loss of intellectual capacity, animal studies have shown marijuana-induced structural damage to portions of the brain essential to memory and learning.