I was under the impression that, because it is addictive, you need more and more to reach the same effect and that eventually, you cannot function in society anymore ......... ?
'Tolerance' is the key word here.
new user -> gets sick / feels nothing
2nd/3rd time user -> gets high
junkie-> uses to feel what he perceives as 'normal'/does not get high
regular use builds tolerance. 'tolerance' is the body's adaptation to the regular/constant presence of the substance. because the user's body is adapting to the presence of the drug, the user has to keep using more to achieve the same high.
the more the substance is present in the body, the more the body (and brain) adapts to the presence of the drug. the more adapted the body has become to the presence of the substance, the smaller the gap between [baseline] and [high]. in other words, the user is still technically getting high, but because the user's mind and body is so used to and adapted to the presence of the substance, the "normal" [baseline] is raised. The 'gap' from [baseline] to [high] is smaller, so the user does not perceive getting as high.
what starts out as 'getting high' ends up being 'getting normal', because of the adaptations that the body makes in order to accommodate the presence of the substance.
non-regular users do not develop tolerance. When they get high, they feel high. hard core junkies on the other extreme are people who's bodies are adapted to the continuous presence of the substance. They don't use to get high, they use to feel normal, because when they don't use, their bodies and brains (initially) are thrown into chaos ('withdrawal') from the absence of the substance (physical dependence).