It doesn't. That happens to some people due to poor methods of smoking. When you roll a joint, put a filter in it (look up RAW filter tips). You can do the same if you prefer blunts, bluntwraps, cigarillos, etc. I use to smoke alot of those and they turned my lips black, but now thanks to filters they are normal & red.
I also do mostly glass (bongs, steamroller, pipes etc) and joints 2nd. Tobacco isn't healthy like cannabis, so mixing isn't really the best idea all the time, although a bluntwrap here or there won't hurt
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