It must be hard being a politician. You go along giving what seem like quite reasonable reasons for doing or not doing things, then suddenly you have to cast about for different reasons when the original justifications don't work anymore.

This looks to me like what has happened recently, with the idea of a tax on sugary drinks. The Government apparently hates this idea, saying taxes don't work, what we need is education. This is in the face of impassioned pleas from a raft of public health experts, who cite unequivocal evidence that taxes work effectively on tobacco - a not unreasonable comparison - and have long worked here.

But until now the idea of a tax on sugar-sweetened beverages (SSBs) was easy enough to bat away by saying there was little evidence this specific tax would have any effect.

Imagine how our ministers must feel seeing the inconvenient evidence last week from Mexico on the effect of the sugary drinks tax implemented in that country.

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Mexicans are the world's biggest consumers, chugging a tooth-aching 163 litres a person per year. Like New Zealand, they also have very high rates of obesity, diabetes and heart disease.

In January 2014, the Mexican Government introduced a 10 per cent tax on SSBs with the goal of reducing consumption.

Preliminary results from a study by Mexico's National Institute of Public Health show the tax has achieved that goal. There was an average reduction in SSB purchases of 6 per cent, rising to 12 per cent by year's end. The greatest reduction was among the lowest socio-economic group, at 17 per cent.

This happened despite some aggressive marketing campaigns by drink companies, including the personalised Coke can promotion seen here over summer. What's more, water purchases increased.

Mexican public health experts are now calling for the tax to be increased to 20 per cent.

When I think about whether we should have a tax on SSBs, I ask myself a question. John Key, if you're reading, I'd encourage you to ponder this, too. Who does this tax hurt?

You could say it hurts people who like to buy sugary drinks. Yes, perhaps it does.

But if it means people think twice before grabbing that brightly coloured bottle and choose something else instead, or even go with plain water, it's hard to see that as a bad thing.

We don't worry about hurting the poor smokers' feelings when the cigarette price goes up.

A tax like this could hurt supermarkets, certainly. And we know they are pretty powerful organisations. But we also know those companies are clever operators.

They are very capable of coming up with new marketing and merchandising strategies.

And I bet they'd be on board with an integrated strategy to encourage other healthy changes in purchasing behaviour, given the opportunity. They are part of our communities, after all. I don't see their profits being under threat for long. The makers of SSBs may feel a little pain from this tax. But again, these companies are filled with clever people and have resources.

They can come up with smarter, money-making, healthier drinks. Judging by recent product launches, they're already on to this.

There's the old argument that a tax would be too hard to administer, that it's too tricky to determine what and how to tax. To that I say, nonsense. There are clever people in Government, too - keep it simple, make a rule and get going.

If it's possible to administer collecting GST on my overseas internet shopping, it's possible to do this.

Lastly, there are people who say this is "nanny state" and why should I pay for other people's lack of willpower?

To you I'd say, you're already paying a big price and you're only going to pay more as the burden of obesity and related disease builds.

As part of a wider strategy, including education, this is a step towards stemming that flow.

And don't you think sometimes we need to do tough things for the greater good, for the long term, not just our own benefit?

Niki Bezzant is editor-in-chief of Healthy Food Guide magazine.