Three more in the seemingly endless stream of saccharin studies have just been published, and the weary and confused consumer would be justified in demanding one final answer: All right, is saccharin dangerous, or isn't it . . . does it or doesn't it cause cancer -- yes or no? Well, the hoped-for answer is not in these studies. But the scientific facts are at least substantially clearer. Individual and institutional judgements about saccharin use can now be reached on the basis of a combination of science and caution.

There are only two ways to discover whether a substance can cause harm to human beings -- and both have severe limitations.The first is by making laboratory studies of inbred animals. While these can reveal a great deal, differences between a particular animal species and humankind do appear -- unpredictably -- from time to time. Sometimes a drug will have little or no effect on rats but produce terrible effects on people: thalidomide is an example. It also works the other way around. The second method is to use human epidemiological studies, which are afflicted by other weaknesses: genetic differences, variations in dose and length of exposure, inaccurate recollection by subjects, air pollution, occupational exposures and many more. In fact, even when both methods are used, identifying and measuring relatively low-level human health risks is, in the words of one expert in this field, "practically impossible."

Within the limits of these uncertainties, laboratory studies and human studies now indicate that saccharin is a weak carcinogen that does not pose a significantly increased risk of cancer for most people when used moderately. However, the largest human study shows a 60 percent increase in the risk of bladder cancer for heavy users -- those who consume the equivalent of four or more dietetic sodas per day. That study also showed a greater risk of bladder cancer for heavy smokers who use a lot of artificial sweeteners than for heavy smokers who don't.

What the studies do not show is almost as important as what they do. Most cancers have very long latencies -- the period between exposure to a carcinogen and clinical appearance of disease. Bladder cancers, in particular, have latencies ranging from 30 to 50 years. Since artificial sweeteners came into wide use in the 1960s, the often-mentioned prospective "epidemic" of bladder cancers could be under way but not yet visible. Nor do the studies identify the effects of saccharin on children, who are physiologically different from adults and who receive a much higher dose (per pound of body weight) from a can of diet soda than does an adult. Finally -- and perhaps most important -- the potentially serious effects on the developing fetus are still unknown.

What all this will mean for government regulators and congressional overseers who are tied up in an emotional (and heavily industry-lobbied) debate over saccharin remains to be seen. However, a prudent course for the individual consumer is relatively clear, and is summarized in this carefully worded advice from Dr. Robert Hoover, who directed the National Cancer Institute's saccharin study: "any use by non-diabetic children or pregnant women, heavy use by young women of childbearing age, and excessive use by anyone are ill-advised and should be actively discouraged by the medical community."