Low-nicotine cigarettes alone don't help smokers quit over the long term, a new study finds.

"We don't know that very low-nicotine cigarettes will not reduce nicotine dependence, but progressively reducing nicotine content of cigarettes in the way that we did, without other means of supporting smokers, did not produce the desired results," study leader Dr. Neal Benowitz, a professor in the departments of medicine and bioengineering and therapeutic sciences at the University of California, San Francisco, said in a university news release.

The study included 135 smokers who had no immediate plans to quit, but were interested in trying low-nicotine cigarettes.

Eighty of them were given cigarettes with progressively lower levels of nicotine, eventually reaching just 5 percent of the level used in regular cigarettes.

The other 55 participants continued using their regular brand of cigarettes. Both groups were told to smoke as desired, and those who were interested in quitting were given manuals on how to do so.

While smokers who used low-nicotine cigarettes lowered their nicotine intake, they were unable to reduce their smoking in the long term, the study found. Only one person in the low-nicotine group ended up quitting during the two-year study. None of the people in the regular cigarette group quit, the researchers said.

They suspect that many of the participants in the low-nicotine cigarette group also used regular cigarettes.

They tested the blood levels of a substance called cotinine that's produced when someone smokes. Levels of cotinine were higher than expected in people smoking the low-nicotine cigarettes, suggesting they may have also smoked regular cigarettes, the researchers noted.

"The results might have been different if regular cigarettes were not freely available, as would be the case if all cigarettes were mandated to be low in nicotine," Benowitz said.

He noted that the reduction of nicotine content in cigarettes nationwide is being considered as a way to make cigarettes less addictive.

But, if higher-nicotine cigarettes are also available, this strategy likely wouldn't work, Benowitz added.

Results were published online July 22 in the journal Addiction.

"Nicotine reduction would work best in the context of public education, easy access to smoking cessation services and the availability of non-combustible sources of nicotine for those who have difficulty stopping nicotine completely," Benowitz said.