A medication used to treat HIV patients will soon be used experimentally in the United States and abroad to see if it might work as a vaccine.

Researchers in seven countries will test the drug, tenofovir, to see if it protects against the AIDS virus and if it's safe to use in uninfected people. The trials will involve about 4,800 people and last two to four years.

"We've had 20 years of [encouraging] abstinence and condoms, and we've not had the effect we wanted," said Dr. Lynn Paxton, who is coordinating a major portion of the program for the Centers for Disease Control and Prevention (CDC).

"If tenofovir is proven effective, it could be available the next day. It could be a very important addition to the arsenal" of prevention efforts, she said.

CDC researchers in San Francisco and Atlanta will enroll 400 gay men at high risk for HIV infection to participate in the trial. The agency also will test the drug on young heterosexuals in Botswana and intravenous drug users in Thailand.

The Seattle-based Bill & Melinda Gates Foundation is sponsoring additional testing in women in Cameroon, Ghana and Nigeria, and in men in Malawi.

Tenofovir, sold under the brand name Viread, has been used for about three years to treat HIV. It lowers the level of HIV by blocking an enzyme the virus needs to reproduce inside the cells it inhabits.

The drug is considered safe and one of the best-tolerated by HIV patients. Scientists want to determine if it's also safe in uninfected people.

The U.S. trial is not designed to test effectiveness, as in several other countries, but will gauge whether people increase unsafe sexual behavior when given a pill that might protect them.

Volunteers will receive extensive counseling against unsafe sex and be given free condoms, along with frequent testing for HIV and other sexually transmitted diseases. Studies show that despite such counseling, some people still engage in risky sex.

In those who do become infected, scientists will measure whether the HIV becomes resistant to tenofovir — though it has not often done this in other HIV patients.

Participants will take a pill daily for two years. Half will receive tenofovir and half a placebo, or dummy pill, and infection rates will be compared at the end of the trial.

Physicians have long used HIV drugs to protect medical workers accidentally stuck by contaminated syringe needles. And drugs given to HIV-infected women around the time of labor lower the risk of mother-to-child HIV transmission by about 50 percent.

Last week, the CDC recommended physicians consider giving the medications to people exposed through sexual assault, consensual sex or intravenous drug use if they seek help within 72 hours after exposure. They were not recommended for people who frequently engage in risky behavior.

Warren King: 206-464-2247 or wking@seattletimes.com