Background: Conscientious objection (CO) in the context of health care arises when a health care professional (HCP) refuses to participate in a certain procedure because it is not compatible with their ethical or moral principles. Refusal to treat in health care includes, in addition to CO, other factors that may lead the HCP not to want to participate in a certain procedure. Therefore, we can say that CO is a form of refusal of treatment based on conscience. Hastened death has become an increasingly reality around the world, being a procedure in which not all HCPs are willing to participate. There are several factors that can condition the HCPs’ refusal to treat in this scenario. Methods: With the aim of identifying these factors, we performed a systematic review, following the PRISMA guidelines. On 1 October 2022, we searched for relevant articles on Pubmed, Web of Science and Scopus databases. Results: From an initial search of 693 articles, 12 were included in the final analysis. Several motivations that condition refusal to treat were identified, including legal, technical, social, and CO. Three main motivations for CO were also identified, namely religious, moral/secular, and emotional/psychological motivations. Conclusions: We must adopt an understanding approach respecting the position of each HCP, avoiding judgmental and discriminatory positions, although we must ensure also that patients have access to care. The identification of these motivations may permit solutions that, while protecting the HCPS’ position, may also mitigate potential problems concerning patients’ access to this type of procedure.