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A "fat tax" on sugary drinks or high-fat foods not only costs taxpayers but is also ineffective at fighting obesity, a Canadian think-tank says.

"The obesity problem is real. There is little evidence, though, that a Canadian soda tax would be anything more than a politically motivated tax, arbitrarily levied on a convenient scapegoat," the Montreal Economic Institute says in a report released Thursday.

Amid a growing number of calls for public-health-based solutions to obesity, such as New York City mayor Michael Bloomberg's ban on 16-oz. servings of sweetened drinks, the MEI says the idea of a tax is misguided.

For one thing, the MEI said, a consumer might choose a cheaper option that is equally unhealthy but does not fall under the tax -- a double-double coffee, with two creams and two sugars, for instance.

Canadians drink more coffee or beer than soft drinks, the MEI said, citing a 2008 Statistics Canada report.

On the other hand, a broad tax like one on all sugar-sweetened drinks might end up including beverages like smoothies or fruit juice.

There is no consensus on how effective a tax would be at combating obesity, the MEI pointed out, in reference to several studies that show it doesn't work.

"Basically, taxing sodas would provide the illusion of addressing the obesity problem. In reality, this tax would have very little effect on those extra pounds," MEI vice-president Jasmin Guenette said in a release.