This narrative review presents theoretical and empirical evidence of common cognitive biases that are likely to influence treatment choices of patients with cancer and other illnesses. We present an overview of common cognitive biases that result from how and when information is presented to patients. We supplement these descriptions with cancer-specific examples or those from other health fields if no cancer-specific examples are available. The results provide compelling evidence that patient treatment choices are subconsciously influenced by both known and unknown biases. Shared decision making is ideal in theory, but in reality, it is fraught with risks resulting from cognitive biases and undue influence of even the best-intentioned physicians and family members. Efforts should be made to minimize these concerns and to help patients to make decisions that their future selves are least likely to regret.