Testosterone Undecanoate
An oral testosterone formulation that bypasses first-pass liver metabolism via lymphatic absorption. The only orally bioavailable testosterone that is not 17α-alkylated, making it significantly less hepatotoxic than other oral steroids. Also available as a very long-acting injectable (Nebido).
Mechanism of Action
Absorbed through the intestinal lymphatic system when taken orally with dietary fat, bypassing hepatic first-pass metabolism. Once absorbed, the undecanoate ester is cleaved to release bioidentical testosterone, which then follows normal testosterone metabolism — aromatization to estradiol via aromatase and 5α-reduction to DHT. Variable absorption is the main pharmacokinetic limitation.
Typical Dosing
⚠ Warning Flags
- •Variable and unreliable oral absorption — must be taken with dietary fat
- •Less predictable blood levels compared to injectable testosterone
- •Estrogen conversion still occurs — monitor E2
- •The only non-hepatotoxic oral steroid (not 17α-alkylated)
Effect Profile
Side Effect Profile
Research Studies
Pharmacology of testosterone replacement therapy preparations
Shoskes JJ et al. · 2016
Characterized pharmacokinetics of oral testosterone undecanoate including its unique lymphatic absorption pathway.
A new oral testosterone undecanoate formulation restores testosterone to normal concentrations in hypogonadal men
Swerdloff RS et al. · 2020
Clinical trial demonstrating Jatenzo restores therapeutic testosterone levels via oral administration.
Testosterone replacement therapy: current trends and future directions
Nieschlag E et al. · 2004
Compares oral vs injectable testosterone preparations including undecanoate efficacy and absorption variability.