Then came back with these sites:

Marijuana | National Institute on Drug Abuse

The Effects of Marijuana

Is Marijuana Harmful? Positive and Negative Effects of Marijuana, Weed - HealthyPlace
The old salt "consider the source" definitely applies here.

Those sites and articles are nothing but pure propaganda with none of the dire "facts" substantiated.

It seems like there mind has already been made up and the negatives of using marijuana outweigh the positives
I think a more accurate assessment would be that they have fallen into the trap of the Prohibitionists who cry that "cannabis is Reefer Madness" - with no apparent hope (or willingness) of digging their way out.