I actually know a lot of pot-smokers (lol imagine that) very few smoke tobacco, and none of them are addicted to nicotine. I occasionally smoke cigars, as do a couple others, but cigars are a lot different than cigarettes. (and by occasionally I mean less than once a month -- I'm not addicted in any lose sense of the word)

I have to say though the smoke from cigars is a lot harder to handle. You can inhale marijuana smoke with no difficulty and even exhale through your nose. Try that with pure tobacco and you'd choke up, breath through your nose and it'll kill your sinuses. They say marijuana joints are like 4 cigarettes as far as smoke weight is concerned but like I said earlier marijuana has very little of the "permanent" sort of tar that cigarettes have (plus you really only smoke half a joint anyway lol). Smoking marijuana does increase your chance of getting colds, and it can cause "smoker's cough" when you get sick -- short term the smoke is bad for you, but unlike tobaco you don't get emphysema or tar buildup. And I believe I read something about scaring (though local) -- I'm not trying to claim it's absolutely 100% safe, but in reality it really isn't all that bad for you.I don't want to derail this thread I think it's obvious I smoke weed but I only started after doing a lot of research myself. I usually don't do things like this without first researching them -- I'm actually borderline anxiety disorder especially with my health. Like I've tried salvia before and that was only after reading just about everything at erowid and the wiki article (salvia is awesome but habitual use is probably bad for you -- both mentally and for your lungs. You don't do drugs like salvia regularly though).

There are short-term side-effects from marijuana smoking. Used in moderation it's a lot safer than alcohol which causes numerous kinds of cancers, health problems, dementia, and even kills (yes, kills) brain cells. Marijuana stays in your body for a very long time compared to other drugs. Conventional wisdom is that you need an entire month of abstinence to clear a drug-test, but if you smoke everyday it can take months before all the thc exits your body. Essentially you can get "burnt out" from smoking weed if you smoke often enough, and being burnt out basically means you're dumber (reduced reaction time, poor short term memory etc).

What I have personally found is that there is conflicting research in peer-review, but there is a pattern as to the long-term side-effects if you read the methods of the study. Studies which allow very little time of abstinence before testing tend to show that marijuana makes you "dumber" in a couple different ways. But all studies which give subjects enough time off the drug show very little correlation on intelligence. Furthermore certain people tend to be more inclined to smoke weed so you can always have a reverse-correlation (which is why I like the Canadian study I'm linking bellow, they measured IQ scores around age 11 and then again 8 years latter, even for the control group, to measure IQ change not absolutely IQ score).

All studies of this sort indicate that heavy use is far worse than light. Heavy marijuana smokers are correlated with other illicit drug-use so it's not just the marijuana there. Plus when you use it on an almost daily basis you get to a point where you're permanently "burnt-out" and this can last sometimes more than 3 months after you quit. Being permanency burnt-out can't be good for your mental health, either. You'd get used to being dumb so to speak and a lack of mental stimulation could alter the process of brain plasticity. So I can see how heavy marijuana use could be bad for your intelligence (just like how heavy alcohol use is associated with dementia and anxiety, but not with moderate use).

Google Answers: Proven Long Term Side Effects of Marijuana Use

Specifically, Current and former marijuana use: preliminary find...[CMAJ. 2002] - PubMed Result

"Assessing marijuana's impact on intelligence quotient (IQ) has been hampered by a lack of evaluation of subjects before they begin to use this substance. Using data from a group of young people whom we have been following since birth, we examined IQ scores before, during and after cessation of regular marijuana use to determine any impact of the drug on this measure of cognitive function. METHODS: We determined marijuana use for seventy 17- to 20-year-olds through self-reporting and urinalysis. IQ difference scores were calculated by subtracting each person's IQ score at 9-12 years (before initiation of drug use) from his or her score at 17-20 years. We then compared the difference in IQ scores of current heavy users (at least 5 joints per week), current light users (less than 5 joints per week), former users (who had not smoked regularly for at least 3 months) and non-users (who never smoked more than once per week and no smoking in the past two weeks). RESULTS: Current marijuana use was significantly correlated (p < 0.05) in a dose-related fashion with a decline in IQ over the ages studied. The comparison of the IQ difference scores showed an average decrease of 4.1 points in current heavy users (p < 0.05) compared to gains in IQ points for light current users (5.8), former users (3.5) and non-users (2.6). INTERPRETATION: Current marijuana use had a negative effect on global IQ score only in subjects who smoked 5 or more joints per week. A negative effect was not observed among subjects who had previously been heavy users but were no longer using the substance. We conclude that marijuana does not have a long-term negative impact on global intelligence. Whether the absence of a residual marijuana effect would also be evident in more specific cognitive domains such as memory and attention remains to be ascertained." (in other words given 3 months abstinence even "heavy" users returned to normal -- btw if you noticed current light users had a 3.2 point IQ advantage over non-users, and a 2.3 point advantage over former users suggesting marijuana may actually have some benefits on intelligence)

Your study gave all users 28 days off the drug which is enough to clear the drug out of moderate and light users (a drug-test would turn up negative) but heavy users would still test positive on a drug test. So naturally it's common-sense that this study reports light use of marijuana has no effect on IQ but heavy use does. When dealing with heavy users we're talking about people who smoke almost everyday for years -- and thc will stay in your body for a very long time after that kind of abuse. In a sense you kind of become high 24/7 though it doesn't seem like that, cause when you light up you just get higher. This is why marijuana has a reverse tolerance effect -- if you already have thc in your body it doesn't take as much to get high where if you start w/ no thc in your body you have to smoke more.

So when you tally it all up after 24 hours all marijuana users show declines in IQ (there is a study that shows just this but I cant locate it). After 28 days heavy users still show a decline in IQ, but not light users. After 3 months neither heavy nor light users show any decline in IQ. There are no studies comparing specific cognitive abilities so it is possible to have long-term residual effects, but specifically to IQ smoking marijuana appears to have no negative consequence given enough time off the drug, and indeed may actually make your smarter.

When looking at this you have to actually go to the scientific sources of information. Many websites claim numerous long-term side-effects from smoking marijuana and cite these studies, but if you look at the actual studies you'll find that there aren't many proven long-term side-effects, and to the extent that there are side-effects they're not noticeable enough to carry statistical significance -- you have to sift through all the junk and misleading claims / political stuff to get to the bottom of the issue (eg the issue of marijuana causing you to be lazy, short-term this appears to be true, but how do you explain the fact that marijuana users do better in their career and make more money than non-users?).

Btw further reading if you're interested, Drug Policy Alliance: Myths and Facts About Marijuana

And a note: marijuana use in high-school is genuinely linked to school drop-outs and bad grades. In college marijuana use does not change how well students do, and in careers smoking marijuana is associated with better success/income. In high-school, especially in the US there's a kind of "wanna be gangster" culture so it appears that drop-outs tend to smoke weed, and that weed itself doesn't actually cause students to do poorly.