The products have been controversial since at least 2002, when researchers at the University of California, San Diego, reported from a large survey that they appeared to offer no benefit. The study did not follow people over time. A government-appointed panel that included nicotine replacement as part of federal guidelines for treatment also came under fire, because panel members had gotten payments from the product manufacturers.

“Some studies have questioned these treatments, but the bulk of clinical trials have unequivocally endorsed them,” said Dr. Michael Fiore, director of the University of Wisconsin’s Center for Tobacco Research and Intervention and the chairman of the panel that wrote the guidelines. Dr. Fiore, who has reported receiving payments from drug makers, said that “there are millions of smokers out there desperate to quit, and it would be a tragedy if they felt, because of one study, that this option is ineffective.”

In the new study, conducted in Massachusetts, the researchers followed a representative sample of 1,916 adults, including 787 people who said at the start of the study that they had recently quit smoking. They interviewed the participants three times, about once every two years during the 2000s, asking the smokers and quitters about their use of gum, patches and other such products, their periods of not smoking and their relapses.

At each stage, about one-third of the people trying to quit had relapsed, the study found. The use of replacement products made no difference, whether they were taken for the recommended two-month period (they usually were not), or with the guidance of a cessation counselor.

One subgroup, heavy smokers (defined as those who had their first cigarette within a half-hour of waking up) who used replacement products without counseling, was twice as likely to relapse as heavy smokers who did not use them.

“Our study essentially shows that what happens in the real world is very different” from what happens in clinical trials, said Hillel R. Alpert of Harvard, a co-author with Dr. Connolly and Lois Biener of the University of Massachusetts, Boston.

The researchers argue that while nicotine replacement appears to help people quit, it is not enough to prevent relapse in the longer run. Motivation matters a lot; so does a person’s social environment, the amount of support from friends and family, and the rules enforced at the workplace. Media campaigns, increased tobacco taxes and tightening of smoking laws have all had an effect as well.