Serotonin is a chemical known to contribute to wellbeing and happiness – often called the happy chemical. It is ubiquitous across the animal kingdom and in fact, its role is complex, associated with appetite, anxiety, temperature regulation, sexual activity and sleep. There is an impending perfect storm of increasing mental health problems and the pollution of our waterways with the selective serotonin re-uptake inhibitors used in their treatment, combined with climate change and temperature stress, which can affect serotonin recognition and biochemical pathways. Pollution, which affects the serotonin pathways in animals, can affect their reproductive and predator avoidance behaviour, and have knock-on impacts for entire food webs, aquaculture and biodiversity. There is therefore an urgent need to better understand the future faces of this happy chemical, and how it could change our world.