Among the good news to come out of the 22nd International AIDS Conference in Amsterdam this past week, were recent study results that prove taking PrEP (pre-exposure prophylaxis) does not affect feminizing hormone levels in transgender women, as reported by NAM AIDSMap.

“These results provide reassurance that you can use PrEP without fear that it will decrease hormones to a suboptimal level,” said Akarin Hiransuthikul of the Thai Red Cross AIDS Research Centre during a press briefing on Tuesday.

The new iFACT study, jointly conducted by several major world health organizations in Thailand, was prompted in part because prior research showed blood concentrations of PrEP drugs in trans women were lower than expected. One previous study, presented at last year’s International AIDS Society Conference on HIV Science, found that some trans women living with HIV are hesitant to use antiretroviral therapy or do not take it as prescribed because of concerns about drug interactions with their hormone therapy. Many HIV-negative trans women may have the same fears around PrEP use.

Study researchers theorized that this decrease in drug levels occurs either because feminizing hormones interact in a way that reduces drug levels, or because trans women have poorer adherence. The latter may occur due to the myth that PrEP will negatively affect their hormone therapy, which Hiransuthikul says trans women may prioritize over HIV prevention.

The iFACT study enrolled 20 HIV-negative transgender women who still had intact testicles and had not received injectable hormones within the past six months. At the start of the study, the participants started a feminizing hormone therapy regimen of estradiol valerate (2 mg/day), a form of the female hormone estrogen, plus the androgen blocker cyproterone acetate (25 mg/day). (Hiransuthikul stressed that feminizing hormone regimens vary in different countries and these results are only applicable to this particular regimen.)

At week 3, the women started taking Truvada (the branded name of PrEP, which contains the drugs tenofovir and emtricitabine). At week 5, they stopped the hormone regimen so the researchers could compare PrEP drug levels on and off hormones, and then resumed at week 8. The women then continued on both hormone therapy and PrEP through week 15.

The study showed that concurrent use of hormone therapy and PrEP did not affect hormone levels. Comparing levels at week 3 (before starting PrEP) and week 5 (while on PrEP), the researchers saw no significant differences in estradiol levels. Also, no changes in testosterone levels occurred. With these findings, they concluded that PrEP did not significantly affect levels of feminizing hormones.

However, in looking at the effect the feminizing hormones had on PrEP drugs, researchers did discover that total tenofovir exposure was about 13 percent lower in the presence of estradiol. (The study did not measure emtricitabine levels.)

This difference was statistically significant, but Hiransuthikul emphasized that even with this reduction, the tenofovir level was still above the target level shown to confer protection. He also noted that it is not known whether this difference in blood levels correlates with tenofovir levels in rectal tissue, where HIV exposure typically occurs.

One other interesting thing researchers noted after analyzing previous studies—although PrEP appears to work well for cisgender (non-trans) women, previous data suggests that they may need to take PrEP more consistently than men to achieve a similar level of protection. This suggests female hormones in general may affect drug levels, and stricter adherence is needed for both transgender and cisgender women.

Since trans women are of the most marginalized and vulnerable populations in terms of HIV, it is extremely vital that these women understand they can safely continue their hormone therapy and protect their health with PrEP.