You can say "the marijuana" only if you are referring to a specific type of marijuana. For example; "White Widow is a breed selected for its high concentration of THC. Some people find the marijuana too strong."
On a slightly different note, you say.. ""some people feel narcotized when/while using marijuana", are you sure that is what you want to say? Surely the effect of smoking marijuana, is to be narcotized (awful word) The meaning being..."to subject to the influence of a narcotic drug".
If you smoke it, you are subjected to it's influence, some may feel the effects stronger than others, but all would be affected.
I live in Holland (where it is not a criminal offence to smoke it), I've never seen anyone smoke marijuana and not be affected by it.