For coffee and tea drinkers, choosing a sweetener — pink, blue, or yellow packet? — became more complicated with the introduction this month of Truvía, billed as “Nature’s Calorie-Free Sweetener.”

Wrapped in green-and-white packaging, Truvía made a splashy debut at Rockefeller Center in Manhattan. Available via the Internet and at select D’Agostino supermarkets, the sweetener, which is derived from a South American stevia plant known for its sweet leaves, is all natural and “a lot like sugar,” according to its manufacturer, Cargill.

Even as Truvía’s fan base grows among the health conscious, critics are questioning Cargill’s marketing techniques and voicing safety concerns about stevia products, concerns the Food and Drug Administration has also raised.

“It’s anything but natural in the sense that it’s been cultivated and processed and marketed by a major food corporation,” the associate director of the American Council on Science and Health, Jeff Stier, said.

According to Cargill, the opposite is true. Truvía’s main ingredient, rebiana, is derived from the sweet leaves of stevia plants. “All we do is take the sweetener out of the leaf,” an agronomist with Cargill, Dirk Reif, said.

Company officials, who said Truvía was developed after it became clear that a “natural” sweetener was “what consumers wanted,” report that they spent the past three years working with the FDA to ensure the product’s safety. “We’re done and we’re safe,” Cargill’s business director, Zanna McPherson, said. Emphasizing how safe the product is, she added, “It’s a lot like sugar.”

In fact, Truvía’s box employs a slogan similar to one used last year by the Sugar Association in an advertising campaign that endorsed its product as “Sweet by Nature.”

Comparisons to sugar have landed other sweetening products in trouble. Splenda, which was initially marketed as a product that was “Made from sugar, so it tastes like sugar,” settled a lawsuit last year brought by the maker of Equal, which challenged the slogan’s accuracy.

A spokeswoman for the Sugar Association, Melanie Miller, said her group has not taken a position on Truvía, although it is watching to see if the Food and Drug Administration will. “Of course we believe that sugar and honey and maple syrup are the natural sweeteners,” Ms. Miller said. Of Truvía, she said, “It’s extracted from a plant, but there are a lot of things that are extracted from plants that are not natural.”

Critics of Truvía called into question Cargill’s research about Truvía’s safety.

“From my standpoint, they’re not giving nearly enough information,” a professor of nutrition at New York University, Marion Nestle, said. Ms. Nestle, who said she avoids all artificial sweeteners, observed that in recent years stevia has gained a loyal fan base.

Mr. Stier of the American Council on Science and Health said Cargill is marketing Truvia “to play on people’s fears about Splenda, Equal, saccharine, and even sugar.” He conceded that although he did not want to like the product, “I happened to think it tasted pretty good.”

Cargill officials said the Sugar Association’s campaign “had no impact at all” on the way Truvía was branded. “So do people think table sugar is not natural? Sugar cane is not packed up in a packet, either,” Mr. Reif said.

Nutrition and diet experts said defining a food product as “natural” is not easy. “I think ‘natural’ is a tricky word,” a spokeswoman for the American Dietetic Association, Keri Gans, a registered dietician, said. According to Ms. Gans, artificial sweeteners are not dangerous if consumed in moderation, and there is a place for them in some diets. But she said there is a misconception concerning how “bad” sugar is. “One cube of sugar is only nine calories; it is not a lot of calories,” she said.

On a recent afternoon, several New Yorkers who were asked if they would try Truvía said they would for health reasons. “I would try it as long as it was natural,” a shopper at D’Agostino’s on Third Avenue in Manhattan, Donna Driscoll, 56, said.

At Dean & Deluca in SoHo, 27-year-old Paola Ruiz, who said she typically chooses Splenda because “it’s supposed to be the healthier of them,” said she limits her use of sweeteners altogether. “I use it twice, maybe three times a week,” she said.