There will almost certainly be some reduction (unless soda becomes a Veblen good, which seems unlikely but not impossible) the question is how much the price elasticity is. I found a scientific paper[1] that suggest that under some ideal conditions I am not sure apply to reality a 10% tax means 10% less less consumption.

However what do they then consume? A switch from diet cola to juice would be bad from a sugar perspective and a switch to milk would be bad from a caloric perspective. Sparkling water is better from both perspectives but it is worse for your teeth than regular water [2] plain tea seems mixed [3] but realistically would you expect most people to switch to plain tea or tea with tons of sugar?

[1]: http://www.sciencedirect.com/science/article/pii/S1570677X15... [2]: http://www.today.com/health/sparkling-water-bad-your-teeth-d... [3]: https://en.wikipedia.org/wiki/Health_effects_of_tea

Why not just drink water? Why does your drink need flavors? And anyway, the sparkling water will still be better for teeth than the soda. Don't let perfect get in the way of good.

Because it tastes better that way? Anyway the relevant question isn't why my drink needs flavour, but what the majority of those who choose to consume less soda with this proposed sugar tax will switch to. And remember they are doing it for economic reasons, not for health.

Sure, as long as you ignore all the costs of people cutting their soda consumption (e.g. costs to soda drinkers that have a healthy lifestyle, loss of utility from switching to a lower-valued activity, redirection of resources to substitutes).

But if you're going to make that assumption, just ban soda, since you no longer recognize any upsides to it.