Roll-your-own cigarettes may be more addictive than factory-mades, a study has suggested.

The New Zealand study builds on research indicating these cheaper smokes are potentially more harmful.

Rollies' potentially greater addictiveness is attributed to their higher tar/nicotine ratio.

While the addictiveness of nicotine is well established, Victoria University PhD graduate Amy Lewis works in the new field of searching for other bodily mechanisms for smoking addiction.

She said yesterday she had found that tar from tobacco smoke contained a number of addictive elements.

Dr Lewis said tar extracted from a range of cigarettes using a smoking machine was applied to human cells to measure its effects on proteins involved in tobacco addiction.

She also showed tobacco smoke increased the amount of a protein, in cells, that helps to mediate the effects of drugs of abuse. This indicates the Mu opioid receptor protein is also a likely tobacco addiction factor.

"The vast majority of work done to date focuses only on nicotine and how it impacts on addictive pathways in the brain," Dr Lewis said, "but my work shows that other components in tobacco also play a big part.

"There are so many different brain pathways all working together to establish and fortify tobacco addiction.

"Even though most smokers say they want to quit, few are successful and nicotine replacement therapies [NRTs] have proven to be remarkably ineffective at helping them break the habit."

However, the Quitline says using NRT can double a smoker's chances of quitting, and the Health Ministry says that on average, people make 14 attempts to quit before succeeding.

Public health specialist Dr Murray Laugesen said that if rollies were in fact more addictive, that would help explain New Zealand's higher rate of smoking than Australia's (20 per cent, compared with 17 per cent).

It was "another dimension to the argument", following the release last year of research showing New Zealand smokers were exposed to much more tar and nicotine than Australians.

"The main thing about roll-your-owns is the price - they have been cheaper. That was the obvious reason why New Zealanders should prefer roll-your-owns. This [the Lewis research] adds a new twist."

The Government moved to reduce the price gap in the tax rise last year by imposing a much greater hike on loose tobacco than factory-mades, but a gap remains. A cheaper brand factory-made cigarette now costs around 63c and a roll-your-own around 43c.

Rollies are typically thinner but people smoke them more efficiently, potentially making them more harmful.

Norwegian research shows that roll-your-own smokers have double the lung cancer risk of smokers of factory-mades.

Dr Laugesen said no data was available yet on smokers' preferences since the tax increases began last April, but beforehand 31 per cent of tobacco consumed in New Zealand was for rollies, up from 13 per cent in 1990.