The Federal Trade Commission said Tuesday that it no longer considers reliable a test for tar and nicotine used for more than 40 years and touted by the tobacco industry in marketing "light" and "low-tar" cigarettes.

The FTC said it may rescind its guidance on tar and nicotine yields that is based on that test. The commission said if the guidance is withdrawn, advertisers should no longer use terms suggesting FTC endorsement or approval of any specific test method.

The test, known as the Cambridge Filter Method, is a machine-based test that smokes cigarettes according to a standard procedure and is sometimes referred to as "the FTC method." The FTC issued its guidance in 1966 at a time when most public health officials believed reducing the amount of tar produced by a cigarette could reduce a smoker's risk of lung cancer. The commission believed that giving consumers uniform information about tar and nicotine yields of cigarettes would help them make informed decisions about the cigarettes they smoked.

But the FTC said Tuesday scientists now believe the test does not provide meaningful information on relative amounts of tar and nicotine people are likely to get from smoking different brands of cigarettes.

The main reason is that smokers often alter their behavior to get the necessary nicotine dosage, the FTC said. The techniques include taking larger and deeper puffs, smoking more and blocking the ventilation holes that may contribute to lower levels of tar and nicotine.

Nicotine acts as a stimulant and is one of the main factors contributing to the addictive quality of smoking. Tar is the residue from burning tobacco and one of the most destructive byproducts of smoking, accumulating in a smoker's lungs.