Sugar is fattening. Cyclamates have been banned. Saccharin leaves an aftertaste and causes cancer in laboratory rats. It seems like you're left without much choice when it comes to sweetening your morning cup of coffee.

Well, consider this: A white powder, 200 times sweeter than sugar, virtually calorie-free, having virtually no aftertaste, created naturally from amino acids, and generally believed to be have been tested for safety more extensively than any other food product in history.

It's called aspartame, and it's here now. Seventeen years after the accidental discovery of the sweetener, G.D. Searle & Co. will begin shipping it to stores nationwide today, in packets bearing the brand name Equal. A national advertising campaign will break later this year, and Searle is hoping to start using the product soon to sweeten everything from candy to soft-drinks..

Aspartame is not perfect. As currently formulated, it loses its sweetness when heated -- which rules it out, at present, for cooking (though not coffee) -- and it's more expensive than sugar or saccharin. But it has the potential to replace saccharin, which was nearly banned in 1977 but won a stay of execution that is now set to expire next August. If that happens, Searle will have the low-cal market all to itself -- competitive sweeteners are years away. There's no telling how many sugar users will switch over to aspartame, although the company has found in test marketing that about half its customers are coming over from sugar. In any case, it seems certain that aspartame's introduction will kick off a war for America's sweet tooth.

By some reckonings, aspartame is a potential $500-million-a-year bonanza for Searle. That would be a hefty addition to the company's sales, which last year were a bit below $1 billion.

Not everybody is enthusiastic about aspartame, though. Some Wall Street analysts are a little skeptical that it can find a market, primarily because of its price. And Sherwin-Williams Co., which makes the bulk of the saccharin sold in the U.S., is diplomatic about its new competitor. "Our experience with cyclamates, when cyclamates were in the market, was that cyclamates expanded the market," a spokeswoman said. "

While aspartame, if successful, would be a good thing for any company, it's the best thing in years to happen to Searle, capping a five-year turnaround of the company wrought by former Defense Secretary Donald Rumsfeld and a management team formed in large part by veterans of the Ford administration.

The Rumsfeld team has revamped a company that was mired in ill-advised acquisitions, too-heavy an emphasis on oral contraceptives, and a research and development vacuum. Searle has cut away ill-fitting businesses, filled holes in its product line by licensing drug formulas from other companies, and begun pouring money into R&D, the lifeblood of a drug company.

Some of those actions are already paying dividends, although the R&D pipeline is still a few years away from flowing heavily. But Searle's immediate future rests on that sweet white powder, aspartame.

"There's no question but that it is important to Searle," says Rumsfeld, who dumps so much aspartame into his coffee that subordinates kid that he'll get low-calorie diabetes. "Certainly it comes at a helpful time. It could never come at a bad time. Having a good product like that come along can never be considered a bad thing."

Still, Searle officials are very cautious about making predictions for aspartame. Although they express all sorts of enthusiasm for the product, it's virtually impossible to pin any of them down on sales predictions. "There are so many variables," Rumsfeld says, begging off the question. "When you mix up reasonable estimates on 13 or 14 variables, you get out on thin ice."

There are indeed a large number of unknowns, even for a new product. One major question is price. Aspartame costs three or four times as much as saccharin and a little more than sugar, and while this has so far proved to be not much of a handicap in test markets, analysts say price-resistance could be a negative factor.

Searle also hasn't yet geared up for full-scale production of aspartame. What's being sold now is a stockpile built up over the years and made for Searle by a Japanese firm. The company is putting the finishing touches on an aspartame-manufacturing plant near Chicago, but analysts say Searle will need much more manufacturing capacity if aspartame is to crack the soft-drink market and other uses.

Then, too, there is the safety issue. And therein lies the reason why it has taken Searle so long to get aspartame to market.

Aspartame has been around for a couple of decades, having originally been developed by another company as a potential anti-ulcer drug. Technically, it's the combination of two amino acids, aspartic acids and phenylalanine, which are found in many other foods.

The discovery that it could be used as a sweetener was pure serendipity: A Searle researcher, violating the first rule of high-school chemistry, licked his fingers while working with the compound and noticed its sweetness. Searle's patent covers only aspartame's use as a sweetener.

Getting government approval, unfortunately, wasn't as easy.

Given the health furor that has surrounded cyclamates and saccharin, aspartame was probably due for a rather stringent approval process anyway. But Searle never expected it to take as long as it did to get clearance to market the product -- especially because, unlike those two sweeteners, it was a naturally based product.

The Food and Drug Administration granted approval to aspartame in 1974, but quickly rescinded the order after objections arose that aspartame could cause brain damage in young children suffering from phenylketonuria (PKU), a genetic disorder that affects about one infant in 15,000. Children with PKU are unable to metabolize phenylalaline -- one of aspartame's ingredients -- which appears in many foods. But it isn't common in sugar or other sweeteners, so Searle has put a warning on Equal packets to alert parents planning their children's phenylalaline-free diets.

It took years to clear up that controversy -- Searle says the problem was exacerbated by a clerical error in some test data -- and the company wound up suing the FDA for approval. When the FDA finally okayed the product last summer, agency officials said they had never tested a product more thoroughly for safety.

With the approval, the company began rushing to put its magic powder on the market. Aspartame will be available under two names: Equal, the "tabletop" product, and Nutrasweet, which is the trade name for the compound itself. The company will sell Nutrasweet to other firms for a wide variety of uses. The first on the market will include Lipton ice tea mix and General Foods' Countrytime lemonade mix. Searle hopes to apply later this month for FDA approval of Nutrasweet as a soft-drink sweetener -- a six-to-12-month process.

The delays in gaining initial approval for aspartame ate up much of the time on the company's patent of the product. Searle must get aspartame to market and establish it before 1987, when patent protection runs out. Although the company is lobbying for the congressional measure that would extend patents on products that undergo lengthy government approval processes, Searle officials aren't betting on getting the extension.

Searle believes it can establish the Equal brand in the time before the patent runs out, but it is more worried that the bulk market -- principally sales to soft-drink makers -- will start slipping away.

Profits from aspartame will be plowed back into the company's other businesses, including a variety of drug lines and Pearle Vision Centers, and will be used to further spur research and development. One area the company has begun research in is genetic engineering, and analyst Mary Yost of Crocker Investment Management Corp. believes the company may be working on a process to use genetic technology to produce aspartame.

Searle officials are quick to point out that aspartame isn't all there is to the company, and they insist that despite the company's new venture into the consumer food market, it's still first and foremost a pharmaceuticals firm.

Rumsfeld, however, doesn't rule out moves into other consumer areas. "I don't think it's inevitable, but life being what it is, when you have something as substantial as Nutrasweet and Equal. . . energetic bright people could come up with things that might relate," he says.