The risk reduction climbed to nearly 73 percent among study participants who took the pill 90 percent of the time, the researchers added.
Both the treatment and the control group in the study also received intensive counseling on safe sex practices and were given condoms (about 500,000 condoms were distributed throughout the course of the trial).
Men in the study reported having had an average of 18 sexual partners in the previous 12 weeks.
Another expert agreed, however, that Truvada is not meant to supplant the condom.
"There has always been a concern that people will see this as an either/or proposition [pill or condom] but it's clearly 'and,'" said Dr. Michael Horberg, director of HIV/AIDS at Kaiser Permanente Health Plan.
"A lot of organizations such as ours have strategized how best to interpret the study results and we totally agree with the considerations of the CDC," he said.
At the time the study was released, the CDC issued "immediate cautions" with similar but less specific recommendations.
This interim report includes more specific information, including getting tested and receiving safe-sex counseling every two to three months while on the medication.
The guidance is intended to fill a gap until formal U.S. Public Health Service guidelines are issued hopefully later this year, Smith said.
"We're working on the first draft of the formal guidelines but, because they involve other Health & Human Services agencies and a public comment process, we don't expect them until later this year," said Smith, who is a biomedical intervention implementation officer at the U.S. National Center for HIV/AIDS Prevention, part of the CDC.
One cloud in the silver lining: A year's worth of Truvada totals more than $
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