This is part of the study:
Anecdotal evidence from patients, as well as those men who have previously used nandrolone from “alternative” sources suggests a relationship with the use of nandrolone (alone, not in combination with testosterone) and ED. Indeed, nandrolone may contribute to the development of ED through two mechanisms: the suppression of testosterone/DHT via negative feedback and the buildup of estrogens.
Numerous studies have shown that DHT is the active androgen involved in maintenance of nitric oxide-mediated penile erections. Castrated rats treated with exogenous testosterone recovered erectile function but, when co-administered with a 5ARi to block DHT production, this recovery was lost (40,41). Moreover, administration of transdermal DHT in aging men resulted in improvement in early morning erections and the ability to maintain erections (42). In fact, one of the most common sexual side-effects of 5ARi’s (described above) is ED (43).
Nandrolone has also been shown to decrease LH, FSH, and endogenous testosterone levels in animal models, indicating a negative feedback loop to inhibit the hypothalamic-pituitary-gonadal (HPG) axis (44). In this context, nandrolone acts as an androgen receptor agonist that is not converted endogenously to DHT (15). As such, it provides negative feedback to the HPG axis to suppresses testosterone levels, further decreasing the available testosterone and DHT, compounding its negative effects on erectile function.