I agree with keeping moderation in what we do. Even too much water can have negative effects on the body. Cannabis is approached in a negative light because of how it has been portrayed in movies, shows, and t.v. in general. As for the addiction of cannabis, it is arguably psychological. The cannabis plant is only expensive because it is illegal, otherwise you can grow it yourself (hardly a dent in your pocket).
To me it seems the pharmaceutical industry pumped a lot of it's resources to keep marijuana off the shelves and paint it in a negative light. In fact, evidence shows, many studies done in the past on marijuana were funded by companies that had major ties with the pharmaceutical industry or were directly funded by them. What I find odd is that as the cannabis plant is slowly being legalized throughout the U.S., newer studies are contradicting old ones. That is not to say that the use of cannabis is the same as it was in the past.
The link you posted says "Frequent marijuana use can have a significant negative effect on the brains of teenagers and young adults." And also, "People who have become addicted to marijuana can lose an average of six IQ points by adulthood." My question is, did this study take into account environmental, physical, and personal factors? Factors such as, were the subjects categorized according to salary/income, location, and were they mentally or physically stable to begin with? Most teens and young adults who turn to drugs (including cannabis) are doing so because they are battling depression and other difficulties. I suppose I'll look into that myself and get back to you. Ultimately the study admits that those affected negatively by regular use of cannabis were in the minority and even then it relies on "maybe."
Keep in mind that your friends and Man of Faiths family member being affected negatively would be the minority according to your study. Here in the States, majority of the people I know (regardless of their position in society) use cannabis regularly. Most of them are extremely ambitious, successful, and have 4 year college degrees (including myself). It's as if those who've never tried cannabis know someone who has been affected negatively by it and most of those who've even tried it once speak positively about it.
I respect that you don't support cannabis, and to be quite honest, nor do I. I don't support the cannabis plant because the plant is being abused in today's society. It is being used with the wrong intentions (leading to psychological addiction).
I wasn't going to reply to this thread till tomorrow because I am extremely busy, but I had to reply today to keep the thread alive. Because I'm in a rush I was not able to post more studies and evidence backing a few of my claims in this response. I will reply with many more links either tonight, or tomorrow when I have time Inshallah. Bare with me.