Abstract Background Informed consent is a basic concept of contemporary, autonomy-based medical practice and facilitates a shared decision-making model for relations between physicians and patients. Thus, the extent to which patients can comprehend the consent they grant is essential to the ethical viability of medicine as it is pursued today. However, research on patients’ comprehension of an informed consent’s basic components shows that their level of understanding is limited. Methods Systemic searches of the PubMed and Web of Science databases were performed to identify the literature on informed consent, specifically patients’ comprehension of specific informed consent components. Results In total, 14 relevant articles were retrieved. In most studies, few clinical trial participants correctly responded to items that examined their awareness of what they consented to. Participants demonstrated the highest level of understanding (over 50%) regarding voluntary participation, blinding (excluding knowledge about investigators’ blinding), and freedom to withdraw at any time. Only a small minority of patients demonstrated comprehension of placebo concepts, randomisation, safety issues, risks, and side effects. Conclusions We found that participants’ comprehension of fundamental informed consent components was low, which is worrisome because this lack of understanding undermines an ethical pillar of contemporary clinical trial practice and questions the viability of patients’ full and genuine involvement in a shared medical decision-making process.