The diamond-shaped little blue pill that launched a million jokes and racked up the hottest first year sales in pharmaceutical history is being prescribed for a new group of patients: women.

Although the use of Viagra, which was approved 18 months ago by the Food and Drug Administration to treat impotence, is not officially approved for women, some doctors are prescribing it in the hope that it will counteract the libido-deadening side effects of antidepressant drugs and the sexual problems that can accompany aging.

So far, however, the use of Viagra for women is grounded in speculation, not science.

"Scientific underpinning? There isn't any," said Irwin Goldstein, a urologist at the Boston University School of Medicine and a leading Viagra researcher who is a proponent of its use by some women. "We're not at the level where we can generate the science yet. There are general practitioners giving it, there are obstetrician-gynecologists giving it, there are urologists giving it, but there are no double-blind placebo-controlled trials" of the drug's effectiveness or safety for women as there are for men.

A spokeswoman for Pfizer, the drug's manufacturer, said that company researchers are analyzing the results of a study of about 500 European women who took Viagra to determine the drug's effectiveness. Those results will be presented next year in a scientific forum, said spokeswoman Mariann Caprino. "Unfortunately we really can't comment beyond this because we can't promote it" for women.

Once a drug is approved by the FDA, however, doctors can prescribe it "off label" if they consider it to be in their patients' best interests.

Studies of Viagra in men conducted by Goldstein and others established that the drug was superior to a placebo in helping men achieve and maintain erections by increasing blood flow to the penis.

Some doctors who prescribe Viagra to women say they believe the same mechanism could help women. Increasing blood flow to women's genitals could improve sexual satisfaction.

But unlike erectile dysfunction in men, female sexual problems tend to be less obvious and more multifaceted. A large national survey by researchers at the University of Chicago found that the biggest complaint among women was low libido, or lack of interest in sex, which 1 in 3 women reported. Other problems included an inability to reach orgasm. It is not clear whether Viagra could treat either of these complaints.

Goldstein said that he considers two types of women to be good candidates for Viagra: those who suffer from a lack of sexual desire as a side effect of antidepressants such as Prozac and post-menopausal hysterectomy patients who complain of dryness, painful intercourse and diminished ability to reach orgasm.

Goldstein said that he and his colleagues have had little success treating younger women with Viagra if their sexual disorders were the result of relationship problems or child abuse.

Whether Viagra has different side effects for women than for men is unclear, Goldstein said. In November 1998, after receiving reports of the sudden deaths of some men taking Viagra, Pfizer strengthened the warning label and recommended that the drug be avoided by men who had a variety of heart conditions or blood pressure that was too high or low.

At the time the warning label was rewritten, the FDA said that it could not be determined whether the deaths were related to the drug, to the patients' underlying cardiac or blood pressure problems, to the physical exertion that occurs during sex or to a combination of these factors.

The same cautions presumably would apply to women, Goldstein said. Women with heart and blood pressure problems would not be good candidates, he said. Users "have to be able to survive the sexual act," he noted.

Doctors are not the only source of Viagra for women. Business appears to be brisk on the Internet. A Yahoo search for women and Viagra yielded more than 37,000 Web pages, many of them hawking untested herbal alternatives, as well as the drug itself. Some of these sites are aimed specifically at "elderly women."

Officials at the FDA said that while the agency is continuing to monitor serious adverse events related to Viagra, it is not publicly releasing that information. Anyone who wants such data must file a Freedom of Information Act request.

Adverse events reported "since the labeling update [have] not been significant enough to take further action," said agency spokeswoman Susan Cruzan.

Pfizer officials say that more than 13.5 million prescriptions for Viagra have been written for more than 6 million patients, two-thirds of whom are American. The drug is approved in 90 countries, Caprino said. Pfizer has reported that roughly $1 billion in sales have been recorded for the drug, which has eclipsed other treatments for impotence.

Currently about 180,000 to 200,000 Viagra prescriptions per week are written by doctors, she added. That's down from a record 300,000 per week in April 1998, immediately following Viagra's approval by the FDA and the ensuing torrent of international publicity.

CAPTION: Magazine ads promote Viagra, which doctors are prescribing for both men and women.