tar
Oh. My. Goodness.
I woke up this morning with an awful taste in my mouth, I went to brush my teeth and I saw my tongue had turned dark brown. Yuk. Yuk yuk yuk - apparently my lungs are starting to clear out years and years of enctrusted old tar thanks to the two-day break from smoking that I've graciously granted them (and that's made my wallet 13 GBP fatter).
It's actually a good thing that I'm currently undergoing one of my celibate hermit phases (this one is lasting a whole week - OK, I went to bed with a flu, does that count?) so that I will not impose my mood swings and tar regurgitation to anyone.