Smokers are the first to tell you how hooked they are.

"I just haven't tried because I know that it's hard and when I'm ready to quit I'll try to quit," said Brian Large.

"I realize that it's unhealthy, but it is a very hard habit to quit," said Tricia Gooden.

The addiction has even been compared to heroin. But heroin and nicotine affect the brain's pleasure center in completely different ways. So, when scientists found a portion of the brain's pleasure center responding to both nicotine and a heroin-like drug in the same way, the results were unexpected.

"We were a little surprised to see an effect that was so similar between these two different classes of drugs," said Dan McGehee, a neuroscientist at the University of Chicago.

McGehee and his team monitored rat brain cells' response to both drugs. They wrote in the Journal of Neuroscience that in a particular brain region important in addiction, both drugs caused hypersensitivity to the brain's pleasure chemical, dopamine, which can greatly enhance feelings of reward.

"We used levels of the drugs that would normally be in the system in an individual that was using the drugs and we see an effect that's identical," said McGehee. "That argues that the strength is, is the same. It certainly supports the idea that nicotine is a very strongly addictive substance."

McGehee cautions the jury is still out on whether smoking is as addictive as heroin. But he hopes the research will lead to new treatments that interfere with the rewards of harmful drugs, while not affecting the rewarding effects of healthy habits.