Sir John Henry Gaddum FRS FRSE (31 March 1900 – 30 June 1965) was an English pharmacologist who, along with Ulf von Euler, co-discovered the neuropeptide Substance P in 1931.

[3] He was born in Hale (now part of Manchester) the son of silk merchant, Henry Edwin Gaddum and his wife Phyllis Mary Barratt.

[5] From 1927–33, Gaddum worked under Henry Dale at the National Institute for Medical Research, and helped develop the classical laws of drug antagonism.

Together with Ulf von Euler, he established the release of acetylcholine in autonomic ganglia.

[4][8] In experiments with lysergic acid diethylamide (LSD), Gaddum explained how it causes mental disturbances by blocking the stimulating effects of serotonin.

[4] In 1929, Gaddum married Iris Mary Harmer[4] in Royston, Hertfordshire.