An injectable, long-lasting birth control method received high marks from women in recent tests but was sent back to the drawing board to improve its reliability, researchers said.

Norethindrone Microspheres prevented pregnancies for up to three months and 96 percent of the women who used it rated the approach "very favorably," the researchers said. But the failure rate was deemed too high, they said.The drug, made by Stolle Research and Development Corp. of Cincinnati, is a synthetic form of the commonly used contraceptive hormone progesterone. It is encased in polymer spheres of varying sizes that dissolve over time after being injected into a muscle.

"It's injected in the back hip," Dr. Gary Grubb of Family Health International of Research Triangle Park, N.C., a non-profit group specializing in contraceptive research, said Tuesday at a meeting of the American Public Health Association.

Grubb and his colleagues surveyed 245 of 1,036 women involved in tests of the new contraceptive at 11 foreign medical centers and four centers in New York, Houston, Minneapolis and Charlottesville, N.C.

The tests were discontinued in April 1989 after scientists found methods for large-scale manufacturing of the steroid led to higher unwanted pregnancy rates at the preferred lower doses. At higher doses, the failure rate was 2.5 percent per year, vs. 4 percent per year at lower doses, Grubb said.

The only side effects were irregular menstrual bleeding, headaches, nausea and weight gain, he said.

It may be five years before large-scale human studies are completed and the Food and Drug Administration can approve the drug, which would be first long-term injectable contraception licensed for American women, Grubb said.

Dr. Louise Tyrer of the Planned Parenthood Federation of America praised the drug. "Any women who wants long-acting contreception without having to remember to do anything, take a pill, check an IUD string or put something in prior to intercourse, is going find this an attractive method," she said.

The FDA is now reviewing a similar approach known as Norplant, which is six matchstick-size silicone rods that are surgically inserted under the skin of a woman's arm. The rods slowly release tiny amounts of a synthetic hormone and can prevent pregnancy for five years.