Here's a few of the thought-processes behind this, and why I personally find them ridiculous:
- "Weed demotivates you"
Absolutely. In a society where marijuana is prohibited, rather than accepted as a form of recreation, users withdraw. They're told all sorts of half-truths regarding the drug growing up, and then personal experience challenges the validity of all those assertions. Naturally, a user who can't let on publically that he uses marijuana recreationally won't find the motivation necessary to continue bettering himself in society. He's already stepped outside the bounds of societal acceptance, the only way to maintain social acceptance is to either stop smoking, or continue smoking and lie about it.
- "Weed makes you paranoid"
Absolutely. When you're treated like a criminal for strolling through a neighborhood rolling pungeant-smelling cigarettes and relaxing, you've got to develop a sense of fear. I mean, you're typically not bothering anybody with your activities, yet discovery puts you in a position to lose your livelihood, personal freedom, and personal effects. The habit's expensive, you can't afford to cavalier about when smoking simply because you've invested a decent amount in the substance itself and any paraphernelia associated. You're fearing for your reputation, your personal effects, and your very liberty itself. Yea... you're paranoid.
- "Weed is addictive"
Well, yea. Everything in this world is addictive. The human brain is wired to respond favorably to things that act favorably. Smoking is relaxing and fun. There is no established link to physically addictive properties of the drug, so that puts it's addiction level on par with several other legal activities: gambling, sex, driving, riding roller coasters, ice-cream, candy, coffee, alcohol, really good massages, exercise, etc.
- "Weed effects short term memory"
Yep. Memory, especially short-term, is a function of the brain. The effect cannabinoid receptors have on the brain is such that they attach to specific neurons for the duration of their stay within the respiritory system. Alcohol functions similarly, except alcohol can go so far as to subtract hours from your remembered life.
- "Marijuana smoke is worse for you than cigarette smoke."
I dunno about this one. Weed smoke just doesn't behave the same as cigarette smoke. Someone who smokes cigarettes frequently is more readily identifiable than someone who smokes weed frequently. The cigarette smoke clings to things it encounters: clothing, lungs, walls, whatever. While marijuana smoke generally does not. Regardless of that, smoking is only one way to induce TCH into one's system. Pot consumption is equally potent to the user at zero of the assumed cost to the lungs.
- "Weed is a gateway drug."
Correlation does not prove causality. Simply because any percieved majority of weed users may then escalate their usage to other drugs does not mean that it was the marijuana use that caused it. It could also mean that a certain predisposition to heavy drug use exists, and since weed is the most readily and easily available of the illicit drugs, it's simply the first manifestation of that predisposition.
I dunno, there's more I could debunk but if I do I'll be late for work and not be able to eat my dinner.