For migraines, it is episodal use for short period of time
That's flat-out wrong. Topamax is used only prophylactically for migraines. That means people are on it for many months or years, and at a much higher dose than for Qnexa. (As with all known migraine prophylactic drugs it typically takes 1-2 months to even start to work, so people don't take it intermittently).