Nonoxynol-9 (N-9) is a common sperm-killer that's often used with condoms, diaphragms, and cervical caps to more effectively prevent pregnancy. At one time, there was hope that N-9 would also prove to be effective at preventing the spread of sexually transmitted diseases (STDs) such as chlamydia, gonorrhea, and especially AIDS (Acquired Immunodeficiency Syndrome).

But the sad truth seems to be that N-9, a detergent-like substance, may actually make HIV -- the Human Immunodeficiency Virus that causes AIDS -- easier to spread.

In test tubes, N-9 does kill HIV (along with some other disease-causing viruses and bacteria). For this reason, many health professionals began recommending N-9 in addition to condoms as a way to prevent HIV infection. And the makers of products containing the spermicide touted its presence.

But this summer, researchers from the Joint United Nations Programme on AIDS delivered some distressing news in a presentation at the 13th World AIDS Conference in Durban, South Africa. They had tested N-9 in a large group of women prostitutes in Africa, dividing them into two subgroups: one using a vaginal gel with N-9, the other a gel without it.

Surprisingly, the women who used the N-9 gel got HIV 50% more often than those who used the placebo gel. For these women, using the spermicide was actually worse than using nothing. The researchers speculated that N-9, which can irritate the vagina when used frequently, caused small openings in the vaginal lining, making it easier for the virus to get into the bloodstream. Once there, the virus could take hold.

The recent study does have its limits. The women in this study made their income from having sex, so they were often using the N-9 gel all day with many different partners. And they were often not using condoms.

This most recent news muddies the waters on whether and when to use N-9. For people who use it less frequently, N-9 gel may not increase the HIV transmission risk the way it did in the study. And it remains an effective spermicide that improves the efficiency of barrier methods in preventing unwanted pregnancies.

Helene Gayle, MD, MPH, who heads up the AIDS and STD efforts for the Centers for Disease Control and Prevention (CDC), says that while N-9 may remain important to pregnancy prevention, it won't be among the anti-STD tools the CDC recommends in the future. (Her letter to doctors on the latest N-9 research can be viewed at www.cdc.gov/hiv/pubs/mmwr/mmwr11aug00.htm

Researchers are continuing the effort to find safe and easy-to-use compounds that might protect against HIV and other STDs. Such compounds, known as microbicides, could help when a condom breaks or slips, or when people can't convince their partners to use them at all. But there's no proven microbicide on the market yet.

Whether for vaginal or anal sex, a condom -- with or without N-9 --is still considered the best protection against HIV infection and other STDs. If you're trying to prevent STD and HIV transmission, N-9 apparently (alone) won't do the trick, and may be harmful, although more research is clearly needed to confirm this. But as Gayle emphasizes, if the only condoms around are treated with N-9, use them.

Condoms -- not the substances used with them -- are still the most important tool for preventing STDs. Remember that petroleum jelly (for example, Vaseline) can dissolve latex rubber and should never be used as a lubricant with latex condoms.