Medical decision-making involves choices, which can lead to benefits or to harms. Most benefits and harms may or may not occur, and can be minor or major when they do. Medical research, especially randomized controlled trials, provides estimates of chance of occurrence and magnitude of event. Because there is no universally accepted method for weighing harms against benefits, and because the ethical principle of autonomy mandates informed choice by patient, medical decision-making is inherently an individualized process. It follows that the practice of aiming for universal implementation of standardized guidelines is irrational and unethical. Irrational because the possibility of benefits is implicitly valued more than the possibility of comparable harms, and unethical because guidelines remove decision making from the patient and give it instead to a physician, committee or health care system. This essay considers the cases of cancer screening and diabetes management, where guidelines often advocate universal implementation, without regard to informed choice and individual decision-making.