I would guess, and it is only a guess, that if this were to be made legal, cigarette smoking would increase as well.
Plus, any substance that alters your judgment and not for the better, needs to be treated with kid gloves.
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I don't think that legalizing pot would in any way cause anyone to smoke more tobacco.
Nicotine gives a very mild high, and usually only with the first several puffs of the day. It also causes craving for more and more nicotine. The tar in the smoke builds up in the lungs and will often cause cancer and will always cause emphizema and bronchitis.
With marijuana, one reaches a stage of high that does not call for any more marijuana. Either you are high, or you are not. And breathing the smoke is intensely irritating and causes fits of coughing and wheezing, which is a good reason not to smoke anymore than the absolutely necessary.
Alcohol is a depressant, and causes malfunction of motor skills. The more one drinks, the more malfunctions it causes. You could say that the good thing about alcohol is that eventually, a prospective drunken driver will not be able to find the key, the keyholke, or possibly the car.
Marijuana causes changes in perception. Time and space are distorted. Sounds and light are distorted, particularly in the peripheral areas. One hears sounds that are not there, occasionally.
No one should drive while either drunk or high, (or under the influence of anything, including a cellphone) but the high driver can, with effort, concentrate and not run into things. The extremely drunken driver finds this a lot more difficult and could also pass out, or be overtaken by the desire to barf.
Marijuana use should be controlled and restricted. But the main reason people have problems with using it is being busted, failing drugs tests and the like. It is much easier to detect in tests, and by the smell than most other, nastier substances, such as opiates, cocaine, meth and Rx drugs.
Acute observer that I am, my views on marijuana were the result of watching people in the 1960's, and not in the US.
I think it should be decriminalized. People should be able to grow their own, and use it recreationally the way many people use beer.
The few lives that might be lost by abuse of pot could easily be made up for by banning casinos, which do far more damage to people than pot, if one wished to do this.
Perhaps the Indians could replace their casinos with marijuana clubs.