Ethically challenging or controversial medical procedures have prompted increasing requests for the exercise of conscientious objection, and caused concerns about how and when it should be practised. This paper clarifies definitions, especially with regard to discrimination, and explores the restrictions, duties, and practical limitations, in order to suggest criteria for its practice. It also argues that a conscientious refusal to treat, where there is therapeutic doubt, is a valid form of conscientious objection. An email survey sent to General Practitioners (GPs), (...) explored the adherence to the suggested criteria, and assessed the outcomes, in terms of acceptability, impact, and difficulties experienced by the GPs, their colleagues and their patients. The results demonstrate high acceptability of conscientious objection, and better outcomes when the suggested criteria are adhered to. The GPs would support education to improve the practice of conscientious objection to a professional level, which could further improve outcomes and acceptability. (shrink)