It takes smoking at least two-and-a-half cigarettes to fill up all of the brain's nicotine receptors and make smokers feel satiated, and the feeling of satisfaction soon wears off, researchers say.

The National Institute on Drug Abuse reported Aug. 7 that researchers using brain-imaging techniques were able to show that while it takes just a single puff of a cigarette to fill 30 percent of the nicotine receptors in the brain, and three puffs to fill 70 percent, filling all the receptors takes longer.

Experts said the study, conducted by researchers at the David Geffen School of Medicine at UCLA using PET scans, indicates that even just a few puffs on a cigarette can be reinforcing enough to drive the urge to continue smoking.

"Although many smokers endorse a desire to quit, very few are able to do so on their own, and fewer than half are able to quit long-term even with comprehensive treatment," said NIDA Director Nora D. Volkow. "This study helps explain why ... The central findings of the study suggest that typical daily smokers need to have these nicotine receptors almost completely saturated throughout the day, which drives the almost uncontrollable urge to keep smoking."

"Imaging studies such as this can add immensely to our understanding of addiction and drug abuse," added Elias A. Zerhouni, M.D., director of the National Institutes of Health. "These findings suggest that drug therapies or vaccines for smoking cessation need to be extremely potent to compete with nicotine, which binds so readily to these receptors."

The study was published in the August 2006 issue of the Archives of General Psychiatry.