Altrenogest is a synthetic progestin used as a contraceptive method in various animal species, including bottlenose dolphins. This is a retrospective study based on the analysis of data collected between 2020 and 2025. Eighteen female dolphins (Tursiops truncatus) from four zoological parks on the Iberian Peninsula were included, treated with the progestin for variable periods. The animals were monitored through ultrasound examinations, behavioral observations, and hormonal assays for progesterone and estrogen. The results showed statistically significant changes in hormone levels, with a significant decrease in serum progesterone and a significant increase in estrogen levels in treated dolphins compared to untreated ones. These findings suggest that Altrenogest inhibits LH secretion, while it does not appear to directly affect FSH, allowing some degree of follicular activity to persist. Despite its demonstrated contraceptive efficacy, prolonged use of the drug was associated with the development of pyometra in four individuals and follicular cysts in three individuals. These results highlight the effectiveness of Altrenogest in reproductive management of dolphins, while also emphasizing the need for careful evaluation of treatment duration and the potential risk of long-term adverse effects.