A 36‑year‑old woman with hormone‑sensitive breast cancer was given the GnRH‑agonist goserelin to shut down her ovaries, but after about a year and a half the drug stopped working and her ovaries started making estrogen again. This shows that long‑term use of drugs like goserelin (and similar peptides such as triptorelin) can sometimes lead to resistance, meaning they may not keep hormone levels low forever.

Breast cancer is the most prevalent cancer known worldwide in women. It is a heterogeneous, phenotypically diverse disease composed of several biologic subtypes that have distinct behavior and response to therapy. Hormone receptor-positive (i.e., estrogen [ER] and/or progesterone [PR] receptor-positive) breast cancers comprise the most common types of breast cancer, accounting for 75% of all cases. This makes endocrine therapy the standardized treatment for patients with ER+/PR+ breast cancer. Drugs that block estrogen receptors or that lower estrogen levels are the mainstay of treatment. High-risk patients benefit from the addition of ovarian function suppression (OFS)/ablation to either an aromatase inhibitor (AI) or tamoxifen. This case report discusses a 36-year-old premenopausal female who presented with an abnormal right breast lump in the upper outer quadrant of the right breast. Due to high suspicion of malignancy, a biopsy was performed which showed features of both lobular and ductal carcinoma with ER and PR positivity, HER 2 was negative. The patient underwent mastectomy with axillary lymph node removal due to concern for multifocal disease. No clinically relevant genetic mutations were present. Oncotype DX breast recurrence score was 16 and no chemotherapy was offered. Due to large tumor size, young age OFS with goserelin 3.6mg/28 days and letrozole 2.5 mg once daily was recommended. After 16 months of treatment, the patient developed a failure of goserelin-induced ovarian suppression. This case report highlights the possibility of the development of hormonal resistance after long-term use of goserelin.