If you are going to make a claim like that you need evidence. Though I am not full out disagreeing with you I would strongly reconsider your wording to be way less all inclusive. Drugs and their interactions in the human mind are not fully understood so to use that type of blanket statement means i can assume you are wrong. I can assume you are wrong because your sentence allows for no exclusions in a population of over 6 billion. Marijuana is most definitely a psychoactive substance and it contains a decent spectrum of compounds in it so it is possible that any one of those over prolonged exposure can have a negative impact on at least one of the members of the population.
I think it is completely inaccurate to tout that marijuana is "safe" it may be "less harmful and socially acceptable (now) then most other things" but that isnt safe or without danger. It is very important that everyone speaks realistically about cannabis and the compounds it creates. It will only harm "the movement" if people say things like this because they can easily be inaccurate and make those who are against it go "look at what those pot heads believe!" rather then creating an argument that not many can deny.
------- my reply-----
When i was growing up, teenager in high school, marijuana was the source of conflict with my parents. That, other then being on probation for 2 years and a misdemeanor, was the only impact marijuana had on me. I was able to quit for probation for over a year while hanging out with people who smoked daily, hell i would roll their blunts because i wanted to see how far i could go. It never once bothered me. It could be argued that i may have done better in school or college if i didnt smoke and drink myself into oblivion every night. In fact i often wonder what could have been if i were sober and going rather then waking up hungover every day.
But i must say my ground of friends in high school were the group that went no where and did nothing. Most still live in my home town most never went on to other drugs but never went on to much at all. Sure the group in question is happy so it must not matter in the end but they definitely filled the void needed for grocery workers and day shift stock people. Some even have kids and wives and moved a town over but collectively they didnt achieve anything higher then you would picture. To me its not a matter of "what drugs you are doing" but who you are how you were raised and what your ideals were. I always tried my hardest and my life is about breaking the minds of those who thought drugs lead to bad things in high school. Someday i will change the world and people will say "can you believe i sold heroin to him" or "yeah i met that dude in the line of the methadone clinic, i knew there was something about him"
In the end we are who we are because we are who we are and not because of what we did. The decision we make are a reflection of ourselves and they do not dictate the path of future choices. But thats just my opinion on it, i knew people who chose drugs over life but i think thats a personal mistake not an influence of marijuana. That isnt to say its safe or anything i just like to put power in the individual