Unfortunately, one answer is very wrong. Marijuana is addictive. Ask any one who uses it regularly to stop and you will see. The main ingredient THC is stored in brain cells and over time will addle your thinking and shorten your life.
For medicinal purpose, it does help people, for instance, my girlfriend has Autism, and she use to take it in pill form until they made it illegal again in our state. Also cancer patients such as my Aunt use it to ease the trouble of the Chemotherapy, and also assists people with eating disorders such as Anorexia. Of course, if people abuse it, it makes you lazy and gives you the munchies, and during the "high time" you think some dumb stuff. It is not addictive unless additives are put in it, like they do CIGARETTES, which are way more addictive and deadlier than Marijuana.
For more information, watch the movie "Reefer Madness". The side effects are the exact opposite