While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of VIVUS (NASDAQ:VVUS) closed up a big 8% yesterday after Cowen and Company upgraded the obesity drug developer from "market perform" to "outperform."

So what: Along with the upgrade, analyst Simos Simeonidis boosted his price target on the stock to $19 (from $13.50), representing about 70% worth of upside to where it sits now. Simeonidis' research suggests that VIVUS' Qsymia is the most effective weight-loss treatment in the market and is gaining momentum in medical circles, giving investors plenty of return potential especially after the drug's weak launch last year.

Now what: Cowen thinks VIVUS' risk/reward trade-off is particularly attractive at this point.

"Our survey points to Qsymia as the obesity agent with the strongest efficacy," Cowen wrote in note to clients. "We also view its retail pharmacy availability and recent board changes as strong positives for Vivus."

So, while VIVUS is still too speculative for conservative investors, Qsymia's positive outlook, combined with the stock's still-beaten down price (off about 50% from its 52-week highs), make it an intriguing opportunity for biotech-savvy Fools.

Fool contributor Brian Pacampara has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.