Nothing...I guess it depends on how you look at it. There is no one answer fits all...meaning we all are wired differently so to speak, and therefore weed will effect each of us in a slightly different way, also marijuana comes in many different strains and chemically each strain will have a different potency and effect. With that said, if you are a productive member of society, meaning you do your best everyday to contribute to the world in a positive way, you are not driving under the influence, or smoking around children or while watching over children...if you are in control of your consumption of marijuana...especially if you have a terminal illness and are on for example chemotherapy...there is some utility for smoking ...I would say that under certain circumstances...there is nothing wrong with smoking marijuana...but then again this is my personal opinion and holds no liability in a court of law