Once you will be walking in Thamel, it will definitely happend to you at least two things:
One, somebody (more then some) will offer you every kind of known drugs, from marjuana to cocaine...Simply walk further and do not listen on them, and there will be no problem.
Another annoing thing that will certainly happend is this: you will be walking and then a guy will stop you asking only a “questionary about Kathmandu” and then he will take you in an agency where his boss will take 2 hours of your time trying to sell you at least one service.
So, again, simply do not listen on anyone, and go further.
This is a very big problem, moreover in Kathmandu.
Everyone will offer you every kind of drugs, simply avoid them and there will be no problem.
They will really annoing you sooooo much and will insist a lot...It is in a way very strange to see how those people sould sell drugs down the sun, as we say...crazy!
There is no reason to get worried about that...simply take it easy...