Doping: how letrozole masks anabolic use and increases endurance
The Wada banned drug found in athlete Rebecca Passler is a hormone therapy that blocks oestrogen production in breast tumours
First case of doping in Milan Cortina 2026: it is the Italian biathlete Rebecca Passler, who tested positive in an out-of-competition check carried out by Nado Italia a few days before the Olympics. The substance to which the 24-year-old South Tyrolean was found positive was letrozole.
But what is it? It is a third-generation aromatase inhibitor used mainly to treat hormone-dependent breast cancer in post-menopausal women: it is the same drug that cost Sara Errani a ten-month disqualification in 2017.
It is not a chemotherapeutic drug, but a hormone therapy aimed at blocking the production of oestrogen, hormones that fuel tumour growth.
How Letrozole Works
Letrozole acts through selective and reversible inhibition of the aromatase enzyme, also known as CYP19A1. This enzyme is responsible for the conversion of androgens (testosterone and androstenedione) to oestrogens (oestradiol and estrone) in peripheral tissues.
Aromatase is particularly active in adipose tissue, muscle, liver and some hormone-sensitive breast tumours. By blocking this enzyme, letrozole drastically reduces the production of circulating oestrogen, depriving hormone-dependent tumour cells of their main growth stimulus.
