Stewart is an environmental toxicologist and the AstraZeneca global lead for the development of innovative science to support future environmental risk assessments and secure the environmental safety (eco-toxicology) of pharmaceuticals; a key pillar of sustainability.

As a comparative physiologist and expert fish biologist, he leads the mode-of-action driven investigations that provide a mechanistic understanding of how human pharmaceuticals may work in animals and therefore predict the risks to wildlife. One aspect of his research is to drive the aquatic safety testing paradigm away from animal testing, investigating alternative approaches such as using cells to build 3D organoid tissues in culture.  His work and commitment to these alternatives has been recognised by AstraZeneca where he is the global lead for the 3Rs (reduction, replacement and refinement) of animal studies and chairs the AstraZeneca awards.

As the bridge between industry and academic partners, Stewart is privileged to work with some of the leading academics and opinion formers in his field, and he was made Honorary Associate Professor in the College of Life and Environmental Sciences at the University of Exeter in 2018.