That's easy, if sales aren't as expected, the stock price will go down. However, the downside is limited because of the structure of the $65M payment & related elements from Eisai. Still, Vivus got cut in half rather quickly because of disappointing sales. Yet if Vivus hit big numbers, it could have doubled.
The risk reward proposition favors longs. Shorts can only gain $8/share, whereas longs have virtually unlimited gains. $16-$20 is not at all out of the question if the drug takes off. If the drug tanks completely like Qsymia did, it should drop to about $6 or maybe even $5/share, but even that is unlikely unless the EU doesn't approve the drug.
Conversely, when DEA provides final classification, and the EU approves, and a buyout comes to light - at would quickly be a 3-strikes you're out for shorts.
The downside on poor sales is not limited. ARNA will get one 65 million dollar payment upon launch and less than 30% of wholesale revenue. That could be a very small number. ARNA wont make a profit for some years, if ever, based on the Eiasi sales projections over the next 5 years