Jackherer.com website is an excellent source. Make sure you make a list of all there arguments and find PROOF against them.

Braincells? Theres only 1 study that was done about this, and it wen't like this.

They forced monkeys to smoke a TON of weed, like more than most people would ever smoke. They basically suffocated them with carbon monoxide, which comes off any plant materlial that produces smoke. Then they killed the monkeys, counted brain cells and compared them to the dead monkeys who didn't smoke weed. Basically, they suffocated the monkeys. There wasn't enough oxygen getting to their brain so they had more dead braincells.

I'm pretty sure I read that on jackherer.com, but I can't find the page where it says that, and cites the experiment.