There's a humongous risk of screwing your mics up with cigarette smoke. The smoke contains, among other things, tar, and the tar sticks to the membrane and kills the highs.
However, that doesn't mean you cant record tobacco or in a smokey joint, it just means you have to be careful. All you have to care for is using some kind of protection (a rubber's not a good idea here ;-)), a regular foam mic-muff'll do just fine, and make sure noone really blows the smoke right into the mic.
Do it once or just a few times, and chances are you will now really notice much difference, depending on just how traind in listening you are. You will, however, probably notice it'll get harder and harder to mix the material as you want to though. But don't let that fool you, over time, all the small and tiny amounts of crap and filth that'll build up on the membrane will eventually have a tremendous impact on it, making it downright bad. Of course it's next to impossible to completely prevent deterioration and build-ups in the mike, but by being aware of what's harmful for it and protecting it from that as far as possible, you can maintain full audio quality for a very very long time :-)