Nicotine dependence worsening in U.S.

Seventy-three percent of smokers in the last few years were considered "highly nicotine dependent" -- compared to 55 percent of smokers 20 years ago.

Nicotine dependence is worsening in the U.S., but experts don't know why.

Studies show the more physically dependent one is on smoking, the more intensive the treatment needs to be to quit - for good.

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