MEXORYL is not the most notorious drug on the black market. Only a few insiders, most of them women, even know its worth, let alone where to buy it. But it is one of the most ordinary substances ever to be bootlegged. Mexoryl SX, made by the Paris-based skin-care giant L'Oréal, is an illegal sunscreen in this country, one that is thought to be particularly useful in preventing wrinkles.

Called by dermatologists one of the most effective filters of all wavelengths of ultraviolet light, Mexoryl has been used in sunscreen lotions sold in Canada and Europe for more than a decade. But the Food and Drug Administration has not yet approved it.

"I started buying it from Canada," one 46-year-old New Yorker said about Anthélios, which she has used for three years. (She insisted that her name not be published because she did not want it publicly connected with illegal purchases.) The Canadian pharmacy Web site feelbest.com sells a three-ounce tube for a little over $20, which is less than half the cost at Cambridge Chemists.

The woman said she finds Anthélios lighter than titanium dioxide sunblocks and less likely to stain her clothes. "I buy it by the case," she said. "It's pretty good stuff."

The demand for Mexoryl is partly driven by one of the strongest motives: vanity. People are getting wise to the idea that UVA rays, less known than sunburn-causing UVB rays, cause classic signs of aging, not only wrinkles but also sagging skin, brown spots and yellow discoloration. And finding a legal sunscreen in the United States that effectively blocks UVA light, which Mexoryl-enhanced products do, is not as easy as it might seem.

Sunscreen labels often advertise "full spectrum" or "broad spectrum" properties, meaning that they block both UVA and UVB rays. But products can make this claim without specifying how well they protect against UVA rays. And because the familiar sun protection factor (SPF) measurements apply only to UVB blockage, consumers have no handy way to gauge the effectiveness of UVA filters.

Part of the problem is that only within the last 10 years have scientists come to understand the biomechanics of UVA damage. "Up until 1995 the thinking was that UVA was not as important as we now know," Dr. Rigel said.

So far the Food and Drug Administration has approved only three ingredients protective against UVA: zinc oxide, titanium dioxide and avobenzone (trade name Parsol 1789).

But Mexoryl seems more effective than any of these at protecting against UVA light. In 2000, Canadian and French researchers slathered six brands of sunscreen and sunblock on the backs of volunteers and exposed their skin to a UV sunlamp for 15 minutes. The product containing Mexoryl (along with avobenzone, titanium dioxide and other ingredients) was more than twice as effective in protecting against UVA light as any of the others. The study was published in the Journal of the American Academy of Dermatology.

Mexoryl's secret is its chemical structure, Dr. Rigel said. "You can achieve much more efficient and powerful and effective protection with this one ingredient, or you can add it to another ingredient and get an incredibly high SPF protection level, all the way up to SPF 90," as well as UVA protection, he said.

The difference between UVA and UVB light is a matter of wavelength. UVA rays come in longer wavelengths (320 to 400 nanometers), so they pass through the outer layer of skin, rather than burning it as do the shorter UVB rays (290 to 320 nanometers). UVA rays penetrate deep into the dermis, or lower layer of skin, where they can break down collagen and other proteins that keep the skin plump and firm.

"That deeper penetration and deeper damage is what we think is really associated with premature aging in the skin," said Dr. Clay J. Cockerell, a Dallas dermatologist, who is president of the American Academy of Dermatology.

The UVA rays can also damage cells and DNA in the dermis, decrease the skin's immunity and generate harmful free radicals. Though the exact mechanisms remain unclear, doctors assume these actions explain why UVA exposure is also associated with skin cancer.

Unlike UVB light, prevalent only when the sun is high in the sky - between 10 a.m. and 4 p.m. during summer - UVA light is virtually inescapable. "It's present in the same amount from sunup to sundown, 365 days a year, totally independent of climate conditions," said Dr. Katie Rodan, an associate clinical dermatologist at Stanford University.

Mexoryl is also very sturdy compared with other UVA filters, which tend to decompose when exposed to sunlight. That may account for Mexoryl's slightly gummy texture, which can be noticeable on the skin long after it has been applied.

It is hard to tell whether Mexoryl will make it to the United States market anytime soon. A L'Oréal spokeswoman would say only that the company has "initiated a process of discussion with the F.D.A. regarding Mexoryl and is continuing to work closely with the F.D.A."

"I take care of some very well-educated people," Dr. Rodan of Stanford said. "Half of them are Berkeley professors. But beyond the SPF number, they don't know anything about sunscreen or what UVA light does. It's like 'in SPF I trust,' but that's so misleading when you consider the whole picture."