People with a certain genetic variation find it harder to quit smoking, possibly because they have lower levels of dopamine in the brain.

A new University of Pennsylvania study adds more to that picture: When smokers with this variant don't get their regular fix from nicotine, they find it harder to concentrate.

This difficulty seems to be one reason that some smokers can't kick the habit, the Penn researchers say.

"Inability to concentrate after quitting is reported by many patients, and this leads them to smoke to reduce these impairments," lead author James Loughead says.

The researchers gave a memory test twice to 33 men and women who smoked at least 10 cigarettes a day: once when they had been smoking as usual and once after they had abstained for at least 14 hours.

When abstaining, those with two copies of the genetic variation had trouble focusing on the test, which involved remembering geometric figures.

Their accuracy did not suffer, because the test wasn't that hard. But without nicotine, those smokers answered the hardest questions about a tenth of a second more slowly. Functional MRI scans also showed less activity in their prefrontal cortex.

"People say their concentration feels fuzzy," says senior author Caryn Lerman, a psychiatry professor and scientific director of Penn's Abramson Cancer Center.

There may be a solution for would-be quitters in this genetic category, which includes 25 percent of the population. They may respond to a drug called a COMT inhibitor, which also is prescribed for Parkinson's disease. Further study is needed before it could be used as a quitting aid, says Lerman, who knows that quitting is no fun. She's a former light smoker herself.

- Tom Avril