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Scientists are still piecing together why that study pointed to failure and the two latest indicate success. One theory is that the women in the earlier study did not take the medication as often as they should have, Paxton said.

Gilead Sciences of Foster City, Calif., is a major producer of AIDS drugs. On Tuesday, United Nations health officials announced the company had agreed to allow some of its drugs to be made by generic manufacturers, potentially increasing their availability in poor countries.