Anything you smoke is bad for your lungs.
Potheads tout THCs anti-cancer properties while not realizing that doesn't even come near to touching the various carcinogens released when you burn it. So yeah, it's better than smoking cigarettes, but it's still stupid.
Secondly, it affects short term memory. This is why potheads are very forgetful, and can't always maintain a conversation...or even recall a phone number they JUST looked up.
Thirdly, it demotivates you. Potheads often tout that doctors and lawyers and other successful people smoke pot. This statement is true, but is only a half-truth. They fail to realize that firstly, these people don't smoke it as much as your average pothead. Secondly, these people are smart enough to pull it off. And finally that these people aren't average potheads, as your average pothead works a dead-end job flipping burgers and taking out the trash.
Of special consideration is also that it has bad effects on the developing brain. If you aren't 20 yet you should avoid it at all costs...no worries, there are still plenty of years to burn those braincells after that point if you want to try it then. But give the poor organ a fighting chance.
I have smoked pot myself for many years. Laid off it when I got serious about school and my GPA went from 2.0 to 3.85. Of the 40 or so people I hung around with back home who were potheads, only TWO of them are successful now...and one of them had to lay off it completely to do so. All the others are doing the exact same thing they were in high school, a majority of them still living with their freaking parents.
That's not to say that pot itself is responsible. Remember after all the Doctors and Lawyers, yada yada yada. It's not so much that pot inherently makes you worthless.
It's that worthless people tend to smoke pot.
Copied and pasted off yahoo answers