You need more and more of a substance to get the same effects, and you need more before you feel an effect.
Self Control Issues
Its difficult to give yourself limits. You'll say you'll only use "so much" but then you can't stop or end up using twice that amount. Or you use it more often than you meant to.
You hide the drug use or the effect it is having on you from others or have a whole new set of friends with whom you do drugs.
You can no longer stop yourself from using the drug, even if you want to. You are still using it even though it's making bad things happen in your life, like trouble with your friends, family, work, or the law.