The euphoria begins with the first drag.

Inhaling the smoke from a cigarette sends nicotine molecules zooming up into the brain within seconds. The nicotine grabs hold of receptors on brain cells, releasing a wave of dopamine and other chemicals that bring feelings of pleasure and comfort.

Brain cells sprout more nicotine receptors the more you are exposed to it that's precisely why quitting smoking is so difficult, said Megan Piper, an assistant professor at the University of Wisconsin Center for Tobacco Research and Intervention.

"You inhale in, and almost immediately you can feel good, you can feel the buzz," Piper told MyHealthNewsDaily.

When you stop smoking, those nicotine receptors don't get activated, so you're not getting as much dopamine as you're used to, which causes feelings of withdrawal, she said.

Scientists aren't completely sure what happens to the nicotine receptors in the brain if no more nicotine is received.

"In theory, what you'd expect to happen is the receptors would just die off," Piper said. But a 2002 study in the Journal of Consulting and Clinical Psychology showed after quitting smoking cold-turkey, feelings of depression and tension remained, even 31 days after the last puff of smoke was taken.

The results indicate the nicotine receptors persist in the brain, even a month after quitting smoking, she said.

Now, smoking addiction researchers are looking for drugs that could take nicotine's place in activating the receptors, to make quitting easier, or that cut down on the brain's number of nicotine receptors, reducing the yearning to light up.

But fixing a smoker's dopamine craving isn't likely to make quitting a breeze. Smoking also triggers a release of other molecules in the brain that make you feel good, including chemicals called endogenous opioids that heighten positive feelings and subdue negative ones, according to 2004 research from the University of Michigan.

Some say quitting smoking is even harder than quitting illicit drugs like heroin, she said, though that hasn't been proven. But it's possible, because the nicotine from cigarettes reaches the brain in six to 10 seconds "that's a fast hit," Piper said, and an immediate reward. With heroin, which is injected, it takes longer to feel the effect.

And cigarettes are easy to get hooked on because you can smoke them in the car, at home or walking down the street, Piper said.

"You can have all these places, then, that are associated with your using the drug," she said. "There are so many more cues" to smoke.