People smoke flavored tobacco in the nargile, and cigarettes all the time.
People do. You (as a tourist) should absolutely not. Drugs are really illegal in Turkey- and even a small bit can get you in way more trouble than you'd like. It is an absolute certainty you do not want to go to prison (even for a night) or deal with irate turkish cops.
Even if you decide to ignore the warning, you'll find it quite difficult to find drugs, as frankly, turks don't really smoke much weed in Istanbul. With the exception of hippies or real rock and roll enthusiasts, most people treat drugs with a lot of suspician. That means that anyone who offers drugs to you (a foreigner), unless you two know eachother well, is a bit of a shady character, and might be a bit of a scammer.
If you want hashish, go to morocco. : (