His daytime job in clinical radiology funded his evening researches in the laboratory, where he developed an interest in carbohydrate metabolism.

Soon after Macleod's untimely death on 16 March 1935, Kosterlitz was awarded the first ever project grant (£50) from the newly founded Diabetic Association.

On 9 March 1937 he married Johanna Maria Katharina Greßhöner, known as Hanna, a friend from Berlin who had arrived in Scotland in 1935.

In 1968, Aberdeen established a new Department of Pharmacology, which was headed by Kosterlitz as professor until 1973, when he became director of the university's drug addiction research unit.

The degree to which an opiate agonist inhibits contractions of the mouse vas deferens, and other tissues like the guinea pig ileum, is highly correlated to its potency as an analgesic.