I've always felt impatient with people who constantly struggle to quit smoking and repeatedly fail. But genetic research on this phenomenon is awakening new sympathies in me—not all nicotine dependence is equal. While kicking the habit may be an uphill struggle for some people, it's more like climbing Mount Everest for others.

Research by Caryn Lerman, Ph.D. and her colleagues from the Transdisciplinary Tobacco Use Research Center at the University of Pennsylvania has shown that—willpower and inner strength aside—part of your battle to butt out has already been lost or won at the genetic level. Dr. Lerman's group has found that genes determine how quickly or slowly smokers metabolize nicotine, and thus how quickly they are driven to light their next cigarette. Genes also determine just how good smoking makes you feel, and on the flipside, how painful it is to quit.

We know that nicotine is a feel-good drug which stimulates the release of opiates in the brain. That's why quitting is so hard. But for smokers with a particular genetic make up, quitting actually reduces activity in a part of their brain—the dorsolateral prefrontal cortex—which impairs their cognitive function, says Dr. Lerman. "This brain connection may explain why they are at such high risk for relapse," she said, adding that, for these smokers, simply lighting up again reverses their brain fog.

For smokers whose genes make them metabolize nicotine fast and develop cognitive problems in withdrawal, kicking the habit becomes an almost insurmountable challenge. But soon, doctors will be able to use a simple saliva or blood test to see which quitters are starting out with this genetic disadvantage and therefore need the most aggressive help.

What kind of help? Well, research by Dr. Lerman's group shows that people who metabolize nicotine slowly are only mildly dependent and have a pretty good quit rate (42 percent) with just counseling or a nicotine patch, while fast metabolizers only have about half the chance (28 percent) with this approach.

"Fast metabolizers are candidates for non-nicotine medications which may be more costly and have more side effects than the patch," she says. Similarly, smokers who get that "foggy brain" feeling from nicotine withdrawal may soon have a medication designed specifically to combat this brain chemistry. Genetics-based tests for smokers are not yet available for widespread use, but this work by Dr. Lerman and others is paving the way. Today only around 5 percent of smokers succeed on any given attempt to quit on their own. By digging deeper into the world of genetics perhaps she'll provide reinforcement to those still in the battle.