I'd rather my taxes go to preventing obesity in the first place then paying for a pill that helps relieve its symptoms (but does not fix the underlying cause), is expensive, and has side effects. Make it harder to life an unhealthy lifestyle, just as we've made it a lot harder to smoke over the past 40 years. Start by taxing sodas and processed foods for one and use the money to subsidize healthier foods and prevention of "food deserts" in lower income areas (given that obesity is highly correlated with low income which is highly correlated with unhealthy diet -- it's hard to stay healthy when all you have nearby are corner stores stuffed with Frito Lay and Coke.
To me it's another example of how we really have our priorities backwards.