A new emergency contraception treatment that causes less nausea and vomiting than a product now on the market has been approved by the Food and Drug Administration, officials said yesterday.

The treatment, which will be marketed under the brand name Plan B, consists of two tablets of levonorgestrel, a synthetic hormone that prevents conception by blocking ovulation or fertilization.

Susan Cruzan, an FDA spokeswoman, said studies suggest that Plan B is as effective as the current product on the market, but has less severe side effects.

A "morning-after" contraceptive technique approved earlier by the FDA involves the selective use of pills normally used for monthly contraceptive protection. A kit containing only the required pills, and sold under the brand name PREVEN, is now on the market. It follows what is called the Yuzpe regimen, which uses four hormone pills, including levonorgestrel, and must be started within 72 hours after unprotected sex.

Studies sponsored by the World Health Organization in 15 countries and involving about 3,000 women showed that levonorgestrel used by itself is also an effective emergency contraceptive and causes less nausea and vomiting.

The studies showed that the use of Plan B reduces the risk of pregnancy from a single act of unprotected sex from about 8 percent to 1 percent. The drug will not terminate an existing pregnancy. The first pill has to be taken within 72 hours of unprotected sex and the second pill is taken 12 hours later.

In addition to nausea and vomiting, Plan B can, in some patients, cause abdominal pain, fatigue, headache, changes in menstrual bleeding, dizziness and breast tenderness.

Plan B will be distributed in the United States by the Women's Capital Corp. (WCC) and initially will be available by prescription through Planned Parenthood clinics. Pharmacists in Washington state also will have access to the drug during its initial distribution.

A WCC announcement said that Plan B will eventually be available in markets nationwide.

WCC is a privately held company that was organized to bring Plan B to U.S. and Canadian markets.