In a piece for Cato, Christopher Snowden discusses the effectiveness (or lack thereof) of soda taxes that seem to be gaining traction worldwide. Snowden's views closely mirror my own. I like the way he framed the relative effectiveness of soda taxes in this passage:
“Whilst the benefit remains forever on the horizon, the cost can be easily calculated; it is simply the amount of money squeezed from consumers by the tax. In New Zealand, for example, advocates claim that a 20 per cent tax on soda would save 67 lives per year and raise $40 million (NZ).[12] Leaving aside the reliability of the New Zealand forecast, this works out as a cost of $600,000 (NZ) for every life that is extended and does not represent good value for money.
Political action on public health grounds is often justified by the costs of unhealthy lifestyles to the healthcare system, and therefore to the taxpayer. The economic costs of obesity are often misrepresented and fail to account for savings to taxpayers, but even if they were more reliable it is far from obvious that additional taxes would relieve the economic burden.[13] For example, the UK’s Children’s Food Campaign recently claimed that a 20 per cent tax on sugary drinks would reduce healthcare costs in London by £39 million over twenty years, but their own figures suggest that the tax itself will relieve Londoners of £2.6 billion over the same period.[14] The cost of the tax will therefore exceed the savings by several orders of magnitude.”
By the way, if you want to see which (out of more than 100) action will produce the biggest bank for your buck, check out the work of the Copenhagen Consensus, which routinely conducts cost-benefit analysis on a whole set of issues. See their list for the most cost-effective actions.