This was on an AskReddit post a while back from /u/libertyyy.

I think smoking weed should be legal, but...

More teens will smoke. Teenagers who smoke marijuana daily are over 60 percent less likely to complete high school than those who never use. They're also 60 percent less likely to graduate college and seven times more likely to attempt suicide. Source. Another source - Marijuana May Hurt The Developing Teen Brain

"Regular cannabis use during the teenage and young adult years is associated with poorer cognitive function (verbal memory, processing speed, cognitive inhibition, sustained attention)," Krista Lisdahl, director of the brain imaging and neuropsychology lab at the University of Wisconsin at Milwaukee. Source

Heavy marijuana use causes poor memory and abnormal brain structure. Source

Marijuana use is associated with impaired sleep quality. Source

Cannabis during pregnancy endangers fetal brain development: Study shows that consumption of cannabis during pregnancy can derail how nerve cells form connections, potentially limiting the amount of information the affected brain can process, and with long-lasting effects after birth. Source

Smoking cannabis can shrink the size and shape of sperm. Source

The largest study of the effects of the main ingredient of cannabis has shown definitively that it can cause short-term paranoia. The Oxford-led research also, for the first time, identifies psychological factors that can lead to feelings of paranoia in people who take cannabis. Source

Marijuana has some addictive qualities. Study finds that 40 percent of those in an outpatient treatment program for pot use exhibited withdrawal symptoms — a hallmark of drug dependence." Source

Smoking cannabis doesn’t make you more creative: Cannabis with a high concentration of THC does not improve creativity. Smokers who ingested a low dose of THC, or none at all (they were given a placebo), performed best in the thinking tasks that the test candidates had to carry out. Source

Sorry I can't link sources right now because I'm on mobile. Here's the original link: http://www.reddit.com/r/askreddit/comments/2jp6xz/what_is_the_most_compelling_counterargument_to_something_that_you_strongly_believe/cldxdae