Taking PrEP alongside oestrogen ‘may limit’ its effectiveness, study warns

A study examining the interaction between pre‐exposure prophylaxis (PrEP) and oestrogen-based treatment taken by transgender women finds hormone treatment may weaken PrEP’s effectiveness.

The study was conducted in Baltimore, USA between 2016 and 2018, and also found the opposite to not be true - PrEP had no adverse impact on hormone treatment.

With the increasingly widespread use of PrEP among transgender people, concerns have been raised about the interaction between PrEP and hormone therapy. For example, in one survey around 87% of transgender women said they were unwilling to use PrEP due to worries that their hormone treatment may be affected. Until recently, transgender women have been underrepresented in the majority of PrEP trials, making evidence on this issue scarce. 

The study, published in November in the Journal of the International AIDS Society, recruited eight transgender women, all of whom were taking consistent but varied oestrogen therapies, and eight cisgender men. All participants were HIV-negative.

Under direct observation, all 16 participants took oral PrEP (a regimen of tenofovir, disoproxil fumarate and emtricitabine) every day for seven days. To measure the levels of antiretrovirals in each participant, blood was collected just before the final PrEP dose was taken, then one, two, four, six, eight and 24 hours afterwards. Colon biopsies were also collected after 24 hours.

The study found that transgender women had significantly lower tenofovir and emtricitabine concentrations compared to the cisgender men – ranging between 24% and 32% lower. 

Although the study did not go on to examine whether these reduced antiretroviral levels stopped PrEP from being effective, the findings have important implications, particularly for on-demand PrEP. 

Rather than being taken every day, as PrEP was in this study, on-demand PrEP is taken before and after sex. This means drug levels are much lower than when it is taken every day – equivalent to two or three doses of PrEP a week. These findings suggest that if oestrogen did lower drug levels, then the risk of PrEP being ineffective considerably increases. 

This leads the study’s authors to recommend against on-demand PrEP for transgender women who are on oestrogen hormone therapy, at least until more rigorous studies can be conducted. 

The study found no evidence to suggest PrEP harms oestrogen levels. Although researchers did not directly observe the transgender participants taking hormone treatment, when taken in conjunction with PrEP-studies such as iFACT that did, this finding adds to the growing body of evidence that suggests PrEP is not damaging to hormone therapy.