Starting today, the government is requiring that women who receive prescriptions for drugs containing estrogen be given a brochure explaining in simple language that estrogen may cause cancer.

Estrogens, a female sex hormone, are used by 5 million American women during and after menopause. Dr. Donald Kennedy, the commissioner of food and drugs, said the government hopes the warning label will cut this use at least in half.

"Estrogens are valuable drugs, with many beneficial medical purposes," said Kennedy. But he said the Food and Drug Administration fears they are being used too frequently and for too long. For women who use estrogen drugs during and after menopause "for longer than a year the risk of cancer of the lining of the uterus is about five to 10 times greater than for women who have never taken estrogens," Kennedy said.

He added that "women who use estrogens after menopause are about two and one half times as likely to develop gall bladder disease requiring surgery."

Kennedy said in a news conference the least necessary use of estrogens is after menopause. He said there was "no good evidence that estrogens are effective in helping to keep the skin soft or to promote more youthful feelings in the post-menopausal woman.Yet they are widely used for such purposes."

The brochure, which drug manufacturers must print and supply to pharmacists and other dispensers of estrogen, advises women to take estrogen drugs for the shortest time possible and in the lowest effective dose for treatment of menopausal symptoms.

The FDA says about 8 million women also receive estrogen as an ingredient in many contraceptive pills, but there is no evidence linking that estrogen use to cancer. However, since 1970 the FDA has required that birth control pills be dispensed with a warning about their possible health risks to women and their babies. Agency officials said they feel that the post-menopausal use of estrogens is the most serious problem.

The most commonly prescribed brand is Premarin; other brands are Hormonin, Estratab, Evex, Menest, Femogen and Ogen, the FDA said.

The agency proposed requiring a leaflet with estrogen prescriptions in July. The original effective date for the brochure requirement was Sept. 20. The date was pushed back after the Pharmaceutical Manufacturers Association and the American College of Obstetricians and Gynecologists challenged the FDA action in court suits.

A federal court in Delaware refused to issue a preliminary injunction against the FDA. Opponents of the brochure requirement also have suits pending in federal courts in Louisiana and Oklahoma.