Varenicline
"I had to get on here and post my intense side effects that I haven't read anywhere as being all that common.  First of all, I'm only on day 5 and still smoking as usual.  But the past couple days I've been in soooo much physical pain its almost unbearable! My back, neck, and knees hurt so bad since starting chantix and this isn't listed as a side effect, but I've never had this problem before and I'm only 31. Also, the fatigue is so bad I'm nodding off while reading, watching movies, even driving! I'm really trying to stick it out and telling myself it's just my body adjusting to a new medication, but I'm not sure how much longer I can deal with this!"