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 are promising, but it is too soon to recommend that people use ginseng, Barton said. A better idea is exercise — the one treatment already shown to help cancer fatigue, she said.
The flaxseed study was aimed at fighting prostate cancer, not treating a side effect. The edible seed has been used for hundreds of years in cereals and breads and is high in beneficial omega-3 fatty acids and in lignan, a substance that can affect hormone levels and perhaps squelch their cancer-promoting effects.
Four groups of about 40 men who were scheduled to have their prostates removed three weeks later were assigned to get either 30 grams of powdered flaxseed, a low-fat diet, both or neither until their surgery.
After the men's prostates were removed, researchers found that tumors had been growing 30 to 40 percent slower in the two groups taking flaxseed, based on how quickly cells were multiplying. Low-fat diets had no effect on this, said Wendy Demark-Wahnefried of Duke University Medical Center, who led the study.
"Our findings are compelling but they're preliminary," she cautioned.
But several doctors said flaxseed is nutritious and seems to have little downside other than a sawdust-like consistency, since it must be used ground or powdered because it has an inedible hull or coating.
Scientists plans to study flaxseed on men with prostate cancer that comes back after initial treatment, and Canadian scientists also are testing it for breast cancer, she said.
The shark cartilage study was done because Congress ordered it. Some very small early studies suggested high doses of it might extend survival of people with advanced cases of non-small cell lung cancer, the most common form of the disease.
Dr. Charles Lu of the University of Texas M.D. Anderson Cancer Center in Houston tested Neovastat, a shark cartilage liquid extract that the Canadian company Aeterna-Zentaris was trying to develop as a regular pharmaceutical product.
All 379 people in the study, which was done throughout Canada and the United States, were given standard chemotherapy and radiation. Half also were given shark cartilage twice a day.
After about four years there was no difference in survival, which averaged 15 months for both groups.