CHICAGO — The first scientific tests of some popular alternative medicine products hint that American ginseng might lessen cancer fatigue and that flaxseed might slow the growth of prostate tumors.

But a big study proved shark cartilage worthless against lung cancer, and doctors said people should not take it.

The research was reported Saturday at an American Society of Clinical Oncology conference.

The ginseng and flaxseed studies are small and preliminary, and specialists warned against making too much of them because the substances tested are not the same as what consumers find on store shelves.

But the results suggest that some herbal remedies eventually may find niches for treating specific cancers, symptoms or side effects. Americans spend millions on these products, which are not approved by the federal Food and Drug Administration, even though no good studies confirm the benefits they tout.

"One of the most common things patients ask me is about these things they have snookered away in their purses" and medicine chests, said Dr. Bruce Cheson, a cancer specialist at Georgetown University Hospital. "They'll come in with big bags of this stuff."

Some "natural" remedies such as laetrile or high doses vitamin C proved not helpful and even harmful for cancer patients once they were scientifically studied, he noted. Some keep chemotherapy from working as it should.

"Just because it is a vitamin or a leafy green does not ensure it does not have some harmful effects," Cheson said.

Herbal products vary widely in their purity and the amount and type of active ingredients. These three federally funded studies used standardized compounds so they could say with some certainty whether they have any effect.

Debra Barton, a research nurse at the Mayo Clinic in Rochester, Minn., tested powdered, four-year-old Wisconsin ginseng root, which is different from Asian ginseng and other varieties commonly sold, to treat the extreme tiredness that most people suffer from cancer or its treatment.

She randomly assigned 282 people with breast, lung, colon and other forms of cancer to take either 750, 1,000 or 2,000 milligrams of ginseng or dummy capsules daily for eight weeks. Neither the participants nor the researchers knew who received what.

One-fourth of those on the two highest doses said their fatigue was moderately or much better, compared with only 10 percent of those on the low dose or dummy pills.

Results of the ginseng study are promising, but it is too soon to recommend that people use it, research nurse Debra Barton said. A better idea is exercise — the one treatment already shown to help cancer fatigue, she said.