Serotonin (scientifically known as 5-HT ) is an inhibitory neurotransmitter found in the brain but also in large quantities in the gut (about 95% of the body’s serotonin is in the digestive tract). An amino acid called tryptophan is a precursor to serotonin. Serotonin is involved in a wide variety of functions including sleep, appetite, digestion and weight; but perhaps most crucially, it has a direct effect on our mood and sense of wellbeing. The most common antidepressant drugs are SSRIs (selective serotonin reuptake inhibitors), which aim to increase the levels of available serotonin in the brain by preventing serotonin neurotransmitters from being reabsorbed. Serotonin is also the primary neurotransmitter signalling between the brain and the gut.Kim, D.Y. and Camilleri, M., 2000. Serotonin: a mediator of the brain–gut connection. The American journal of gastroenterology, 95(10), p.2698. Sissons, C. (2018). How to boost serotonin and improve mood. [online] Medical News Today. Available at: https://www.medicalnewstoday.com/articles/322416.php [Accessed 20 Nov. 2018]. WebMD. (n.d.). What Are SSRIs? [online] Available at: https://www.webmd.com/depression/ssris-myths-and-facts-about-antidepressants [Accessed 20 Nov. 2018]. Integrative Psychiatry. (n.d.). Serotonin and Serotonin Deficiency. [online] Available at: https://www.integrativepsychiatry.net/seritonin-and-seritonin-deficiency.html [Accessed 20 Nov. 2018].