The nicotine gum and patches that millions of smokers use to help kick their habit have no lasting benefit and may backfire in some cases, according to the most rigorous, long-term study to date of so-called nicotine replacement therapy.

The study, released Monday, followed nearly 2,000 people over a period of years, and is likely to inflame a long-running debate about the value of nicotine alternatives. In medical studies, the products appear effective, making it easier for people to quit, at least in the short term. Those earlier, more encouraging findings were the basis for federal guidelines that recommended the products for smoking cessation.

But in surveys, smokers who have used the over-the-counter products, either as part of a program or on their own, have reported little benefit. The new study followed one group of smokers to see whether nicotine replacement affected their odds of kicking the habit over time. It did not, even if they also received counseling with the nicotine replacement.

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