"I think it's essential for a lot of people that are out there having unprotected sex out and that now they can start taking this pill on a regular basis and protect themselves," said Alonso.
A new international study on healthy gay men showed daily doses of Truvada cut infection risk 44 percent when given with condoms, counseling and other prevention services. Men who took their pills most faithfully had even more protection, up to 73 percent. Truvada stops the virus from replicating.
"If we put someone on drugs that we're currently using for treatment and they take them consistently, all the time, they can make themselves not contract HIV, even in very high risk sexual situations," said Craig Thompson, the executive director of AIDS Project Los Angeles.
The results are "a major advance" that can help curb the epidemic in gay men, said Dr. Kevin Fenton, AIDS prevention chief at the U.S. Centers for Disease Control and Prevention. But he warned they may not apply to people exposed to HIV through male-female sex, drug use or other ways. Studies in those groups are under way.
Thompson says it's a great step forward for AIDS prevention, but he worries it might give some a false sense of security and says Truvada should not be used in place of condoms.
"I don't think we want to just throw out condoms because condoms are very, very cheap and we need to make sure that that's the number one thing that happens," said Thompson.
Because Truvada is already on the market, the CDC is rushing to develop guidelines for doctors who want to use it to prevent HIV.
As a practical matter, price could limit use. The pills cost $5,000 to $14,000 a year in the United States.
Whether the government or insurance will pay for the drug remains one of the tough issues that still needs to be worked out.
"I think there's going to be a lot of interesting questions about should we approve and pay for a strategy like this where people know something else, which is that condoms are absolutely 100 percent affective," said Thompson.
The other drawback is adherence. It has to be taken every day for it to work. But with 56,000 new cases of HIV infections diagnosed every year in the U.S., Alonso is hopeful it will prevent others from getting AIDS and maybe someday lead to a cure.
"In the back of my mind I have always thought that I'm going to live to see the cure and I think this is a big step," said Alonso.
About 20,000 people are part of drug trials around the world that are testing Truvada or some of the components of the drug. Right now, CDC scientists are reviewing studies to see whether or not HIV patients currently getting placebos should be switched to Truvada.
The company that makes Truvada hasn't made a decision yet as to whether or not they'll be seeking FDA approval to market the drug as a preventative.
The Associated Press contributed to this report.