Success at quitting tobacco can be predicted from the way a smoker's body processes nicotine, a study has found.
Measuring the rate of nicotine breakdown can help determine the best strategy to be adopted for a particular individual, experts believe.
The research shows that "normal" nicotine metabolisers - who make up 60% of smokers - might be better off using a drug than nicotine patches.
Those classified as "slow" metabolisers achieve similar quitting success with a patch while avoiding medicinal side effects.
Scientists compared the effectiveness of nicotine patches or the drug varenicline in a study involving 1,246 smokers who wanted to quit.
Of the participants, 662 were slow metabolisers of nicotine and 584 normal metabolisers.
Professor Caryn Lerman, from the University of Pennsylvania, US, who co-led the study published in The Lancet Respiratory Medicine journal, said: "As many as 65% of smokers who try to quit relapse within the first week.
"Our findings show that matching a treatment based on the rate at which smokers metabolise nicotine could be a viable clinical strategy to help individual smokers choose the cessation method that will work best for them."
Although tobacco contains many harmful substances, it is nicotine that smokers are addicted to.
Nicotine cravings occur when levels of the alkaloid compound drop, taking away the "feel good" sensation it imparts on reward centres in the brain.
A blood marker of nicotine clearance known as the nicotine metabolite ratio (NMR) can provide a measure of the rate of nicotine loss, depending on a person's genetic make-up.
Nicotine levels in the body drop more quickly in normal metabolisers than in slow metabolisers.
The study showed that after 11 weeks of treatment, normal metabolisers taking varenicline were twice as likely not to be smoking as those using a nicotine patch.
They were also significantly more likely to still be avoiding tobacco six months later.
In slow metabolisers, both methods were equally effective, but patients taking the drug reported more side effects.
Co-author Dr Rachel Tyndale, from the University of Toronto, Canada, said: "To optimise quit rates for all smokers whilst minimising side effects, our data suggest treating normal metabolisers with varenicline and slow metabolisers with the nicotine patch.
"What is more, it is feasible that a point-of-care blood test to measure the rate at which nicotine is metabolised could be developed and implemented in clinical practice."
Commenting in the journal, British experts Jennifer Ware, Neil Davies and Marcus Munafo, from the University of Bristol, said: "The results ... are an important scientific advance. Should the findings be replicated, they might lead to changes in clinical practice through the implementation of prescriptions stratified on the basis of a biomarker test."
Varenicline, marketed in the UK as Champix, partially blocks the molecular receptors in the brain that are stimulated by nicotine.
Its side effects are not normally serious, the most common being mild nausea.
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