http://blog.euromonitor.com/2011/08/plain-cigarette-packaging-and-the-law-will-it-work.html#more
On the other hand, a year after the introduction of graphic health warnings, smoking prevalence in New Zealand has fallen from 25% to 20%. Although the warnings are not regarded as the only factor impacting sales they are regarded as significant and evidence that smokers are asking for packs with less disturbing warning pictures on them is taken as supporting this.
The Euromonitor International view is that plain packaging would be a massive blow to the tobacco companies and would also have an effect on prevalence (because tobacco brands are so iconic and traditional), albeit not a very significant one. Plain packaging could have a much greater effect on major cigarette brands and the inclination of smokers to buy them, thus its widespread adoption would probably mean we would see the proportion of international brands in the product mix