Thousands of clinical trials are conducted globally each year. Yet, little is known about their environmental impact. This paper presents the results of a high-level literature review of the carbon footprint of clinical trials. Five papers were identified and their contents summarised qualitatively. All papers were authored by UK researchers. Carbon footprint metrics from 14 trials were presented in carbon dioxide equivalents (CO2e). Emissions were broken down by three broadly defined clinical trial activities: operations, travel, and supplies. Recommendations for carbon reduction are discussed. The review showed a dearth of publications on greenhouse gas emissions generated by clinical trials. More work in this area is needed to achieve sustainable, low carbon clinical research.