Endometriosis, a chronic gynecological disorder associated with pain and infertility, is a common condition affecting approximately one in ten women. Up to 50% of patients with endometriosis have ovarian endometriomas (or chocolate cysts). These cysts directly impact fertility and ovarian reserve (ie. ability to have children) and can cause additional symptoms in women such as added pain, discomfort, and surgical emergencies (cyst rupture, or more rarely twisting). While endometriomas tend to require surgical excision as a solution, medical management with a variety of medications has been shown to be effective in reducing their size. Medical management (ie. medications and treatments that don't involve surgery) to reduce cyst size can help relieve symptoms either as a long term solution, before fertility treatments, or temporarily until surgery can be offered. Because the COVID-19 pandemic caused significantly reduced access to surgery and resources, medical management has become important for relief of the overburdened healthcare network. The purpose of this study is to see how effective Estetrol/drospirenone, a combined oral contraceptive (COC), is in the reduction of ovarian endometriomas after a 3- and 6-month period of treatment. This single arm interventional study will recruit women 18 years or older with an ovarian endometrioma of at least 3cm, who are seeking a hormonal treatment for their endometrioma(s). Consenting participants of the study will take Estetrol/drospirenone once daily, orally, for a 6 month duration. An ultrasound assessment of ovarian endometrioma(s) will be performed before starting the drug (0 months), and will be repeated at 3-months and 6-months time. At each of these hospital visits (0, 3 \& 6 months), participants will have their weight and blood pressure measured, and they will complete questionnaires regarding their endometriosis symptoms, incidence of amenorrhea, compliance and incidence of any adverse effects.