Tea, to China's 18th century emperor Chien Lung, was more than a whistle-wetting pick-me-up: It was "that precious drink which drives away the five causes of sorrow."

		Tea Tips Here's how to get the most out of green tea: Don't use boiling water: It can make green tea taste bitter. Cool boiled water for one to three minutes before adding leaves. Allow tea to brew for several minutes. Loose tea needs more time than tea bags. Antioxidants degrade over time, so if you make iced green tea, drink it within 24 hours. Steer clear of green tea supplements. They may not provide the same benefits: A 2005 study found that mega-doses of green tea extract actually helped tumors grow. Check the label if you buy bottled green tea drinks. They may contain mainly sugar and not much green tea.

Western businesses are banking on our buying into Chien Lung's sentiments. In addition to selling a cornucopia of loose green teas, they have distilled the brew's essence and added it to health bars, supplements, diet aids, gum, soft drinks and skin creams -- even, in Asia, to Kit Kat candy bars.

Green tea is good for us: That mantra has been chanted in the West since the early 1990s, when studies reported that the infusion, sipped for centuries in China and Japan, appeared to help fight off cancers when drunk by lab mice or rubbed on their skin. Enthusiasm intensified after other studies revealed that green tea contained certain chemicals with cancer-fighting clout. Scientists rolled up their sleeves to figure out how it works.

Today, green tea imports are soaring.

So confident was one doctor-turned-green-tea businessman that in 2004 he petitioned the Food and Drug Administration to permit green teas to sport cancer-fighting claims on their packages.

The FDA's response: tepid. At best.

In June, the agency ruled that there was "no credible evidence" green tea fights cancers of the stomach, lung, colon, esophagus, pancreas or ovary. The agency acknowledged that the evidence for tea fighting breast or prostate cancer was somewhat better, although it also said the link was "highly unlikely" because the evidence on humans wasn't conclusive enough.

Scientists say that, despite the unanswered questions, green tea still shows promise, not only as a potential cancer-protector but also against other health threats. But they also are mindful that what works in the lab doesn't always pan out in humans.

"You can build your case in cell studies and animal studies but ultimately you have to do it in humans or you can't make a case that it works," says Balz Frei, a professor of biochemistry at Oregon State University.

Green tea is made from the dried leaves of Camellia sinensis, an evergreen shrub native to Asia.

The plant is harvested and treated in different ways to produce green tea or black tea.

Green tea is made by steaming the crushed leaves shortly after harvest, destroying enzymes so that chemicals aren't oxidized very much.

Leaves used for black tea ferment for days before they're heated, causing the leaves to blacken and many chemical changes within them.

Those processing differences may be medicinally important. Both types of tea are abundant in certain antioxidant chemicals called flavonoids, which obstruct the action of cell-damaging free radicals. Green tea, because it doesn't ferment, has much higher levels of a group of flavonoids called catechins. A potent catechin, epigallocatechin gallate, or EGCG, is three to four times more abundant in green tea than black.

EGCG isn't the only thing having an effect. Caffeine is probably providing the lion's share of protection in the case of the skin cancer experiments, and plays a big part in the lung ones, says green tea researcher Chung S. Yang, who chairs the chemical biology department at Rutgers University in New Jersey.

But what happens to humans when they drink tea? Such studies, because they're usually not done in controlled groups, are tricky to interpret, partly because it's hard to measure how much tea people drink, and partly because tea-drinkers do a lot of other things. For example, it's common in China that men who drink a lot of tea also smoke, Mr. Yang says.

The few human studies that have been done have produced mixed results. But despite the complexities, some studies do look good, scientists say.

For example, in an article published in the International Journal of Cancer in 2003, scientists looked at the eating and drinking habits of more than 1,000 Chinese-, Japanese- and Filipino-American women in Los Angeles. They reported that women who drank green tea had a 43 percent lower risk of getting breast cancer compared with women who drank no tea.

But skepticism remains. Like the FDA, the American Cancer Society also concluded that more research is needed to show that green tea helps prevent cancer, and many other scientists concur.

Despite the scientific uncertainty, it still makes sense to drink green tea, says Jeffrey Blumberg, who directs antioxidant research at Tufts University's Human Nutrition Research Center on Aging.

The FDA, because it was responding only to one doctor's specific petition, looked only at green tea's alleged cancer-fighting clout. It didn't consider heart disease, Alzheimer's or hypertension. "Human evidence for a cardiovascular benefit is much stronger than for cancer," Mr. Blumberg says.

Tea has been shown to do a number of heart-healthy things. In human studies it has relaxed artery walls, potentially lowering the risk of high blood pressure, heart attacks and stroke. It reduces a process that clogs arteries -- the oxidation of LDL, or "bad" cholesterol -- and lowers rates of atherosclerosis in animals. It increases levels of HDL, or "good" cholesterol.

Population studies also are intriguing: Last year, for instance, a Chinese study published in the Archives of Internal Medicine reported that people who drank green or black tea had a 46 percent to 65 percent lower risk for high blood pressure.

As for fighting cancer, other human clinical studies are under way in the U.S. If these find evidence that green tea prevents cancer, tea producers say they will petition the FDA again.

If stronger evidence came to light, "we'd re-evaluate the evidence," says an FDA spokesman. "Science evolves."