The medication, called PrEP, is designed to prevent HIV infection in people considered to be at higher risk for HIV, including men who have sex with men, IV drug users and sex workers.
PrEP is highly effective at preventing infection with the virus, reducing a person's risk for getting HIV from sex by about 99% when taken as prescribed, according to the U.S. Centers for Disease Control and Prevention.
However, research suggests that the drug works less well in women, although the exact reasons for this remain unclear.
"The data in this study indicate that the vaginal microbime can impact the efficacy of PrEP," study co-author Nancy Klatt told UPI.
The vaginal microbiome constitutes the bacteria -- both healthy and unhealthy -- living on the vaginal wall in all women. Organs such as the lungs and the gastrointestinal tract also have microbiomes.
"We should consider this impact when choosing drug candidates to move forward in women," said Klatt, a professor of surgery at the University of Minnesota in Minneapolis.
Healthy Lactobacillus bacteria in the vagina are critical for women's health, but the accumulation of additional bacteria can imbalance the vaginal ecosystem, according to Klatt.
For this study, Klatt and her colleagues collected vaginal swabs from 44 women ages 18 to 45 taking a PrEP regimen to identify bacteria colonizing their vaginal microbiomes.
Of the women, 15 had bacterial vaginosis, or inflammation caused by the overgrowth of bacteria naturally found in the vagina, and lower concentrations of PrEP in their samples. This, researchers said, likely impacts efficacy of the drugs in these women.
They found that the bacteria associated with bacterial vaginosis -- but not healthy Lactobacillus bacteria -- metabolize PrEP drugs and may potentially reduce their efficacy due to reduced levels of the preventative drug.
The antibiotic metronidazole is typically used to treat bacterial vaginosis, but many women experience recurrent infection even after taking the drug, Klatt said.
"Women's health is vastly understudied and, given disparities in efficacy of HIV prevention between men and women, it is critical to better understand biology in women and improve HIV prevention options for women," she said.
"More studies and increased treatment options are critically needed."