The smoking rate for U.S. adults dipped last year after a seven-year stall, a new government report says. It's too early to tell why.

Smoking rates have declined steadily for decades, according to the report by the Centers for Disease Control and Prevention (CDC). Last year's decrease – down to 18% – ends a smoking rate stall that hovered at 20% to 21% for more than seven years, then froze at about 19% in 2010 and 2011.

"We are a long way from the end game on tobacco use," said Thomas Novotny, professor of global health at San Diego State University. "It is too early to declare victory."

Smoking is the leading cause of preventable illness and death in the U.S., accounting for one in five deaths and direct medical costs of as much as $73 billion a year, the CDC says. It causes more than 80% of all lung cancer deaths and coronary heart disease, the leading cause of death in the country.

It's unclear what caused last year's smoking rate to decline. Historically, rising taxes on tobacco products, smoking restrictions and mass media and school-based educational campaigns have helped push down the smoking rate, said Joshua Yang, a tobacco control researcher at the University of California-San Francisco.

The preliminary report did not include data on teens. The rate was about 9% for adults over 65 compared with about 20% for all other adults. Men are more likely to smoke than women across all age groups.

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