Sir John Henry Gaddum FRS (31 March 1900 - 30 June 1965) was a British pharmacologist. He was educated at Rugby School and Trinity College, Cambridge. He became professor of pharmacology at the University of Cairo. Subsequent to this he took up a chair at University College London. He later worked at Porton Down. He is best remembered for his classic work "Gaddum's Pharmacology". He is the first scientist postulated that 5-HT might have a role in mood regulation[1].