Weed changes the shape of your brain if you smoke enough, even if it is 'moderated' amounts, Because the part of your brain that reacts to 'Endorphines' or the reward feeling that you get from having sex or eating chocolate or exercise begins to grow and you will want more and more of it. This is the start of the downward plunge. Most people develop cycotic illnesses because of this, schitzophrenia and hallucinations to name a few, so if you smoke cannabis even as a youth and stop for several years, you are at a great risk of becoming re-addicted to the drug. So drugs and alcohol don't mix well with aviation.