Roger Bouillon focuses on Internal medicine, Endocrinology, Vitamin D and neurology, Vitamin and vitamin D deficiency. His Internal medicine study frequently links to other fields, such as Osteocalcin. The various areas that he examines in his Endocrinology study include Calcium metabolism, Parathyroid hormone and Calcium.

His work in Vitamin D and neurology covers topics such as Randomized controlled trial which are related to areas like Intensive care unit and Clinical trial. Roger Bouillon works mostly in the field of vitamin D deficiency, limiting it down to topics relating to COPD and, in certain cases, Exacerbation. His biological study deals with issues like Critical illness polyneuropathy, which deal with fields such as Insulin and Diabetes mellitus.