I was wondering the exact same thing. If I had to try to guess (and it is only a guess) I would look at VVUS that was down 4.40% today on higher than average daily volume. I'm not sure why VVUS was down, except maybe more bad news on lagging script numbers (I'm guessing). If that is the case, that VVUS script numbers aren't exploding, but are just creeping up, then it could make the market skeptical about obesity drugs, which would weigh on ARNA until ARNA can prove that their drug will be different. That would explain the drop in VVUS today and why ARNA is just treading water.
Once ARNA gets final DEA classification, and can start selling the drugs outright, then everything will be driven by their script numbers. If they have high volume out of the gate, this stock could double in a week. Until then though, people are going to be skeptical, based on the problems that plagued VVUS (which I think that ARNA has addressed and has a much better launch plan). Only time will tell.
Negative news stories on Qsymia sales (two today, including one in the WSJ) tend to have a knock-down on ARNA because there's a perception, generally a false one, that if one drug sells badly, so will the other one. Plus, with another weekly OpEx looming, the manipulators will be trying to keep the PPS below 9.0 on Friday. These are short term factors, but enough to kill any rally in the stock today, in my opinion.