Truvada (emtricitabine/tenofovir) as pre-exposure prophylaxis (PrEP) against HIV takes an estimated seven days to reach full efficacy and may protect for nearly a week afterward, the National AIDS Treatment Advocacy Project (NATAP) reports. But those taking PrEP should not assume these are hard facts at this time. Presenting their findings at the 15th International Workshop on Clinical Pharmacology of HIV and Hepatitis Therapy in Washington, DC, researchers conducted an analysis of 11 men and 10 women who took daily Truvada as PrEP for 30 days and then, after stopping the drug, remained in an additional 30 days of follow-up.

The researchers collected peripheral blood mononuclear cells (PBMCs) on the participants' first day of Truvada treatment as well as on days 3, 7, 20 and 30 of treatment, and then they collected samples 5, 15 and 30 days after the participants stopped the drug. The investigators also took rectal biopsies of the participants in order to collect mononuclear cells.

After seven days of treatment, a total of 87.6 percent of the participants reached the blood level of Truvada that the iPrEX study calculated was necessary to reduce the risk of contracting HIV by 90 percent. Eighty-six percent saw the blood level of the drug stay at this concentration for two days after stopping treatment.

The researchers calculated a 99 percent risk reduction for contracting HIV seven days after beginning Truvada, as well as a 95 percent risk reduction for six days after ending treatment.

The researchers caution that these findings are preliminary and this is far from the last word on the matter. Anyone who advises people on PrEP should not tell them that the seven-day time to full efficacy and the six-day tail period of efficacy apply to all.

To read the NATAP report, click here.