A. J. Clark was a British pharmacologist, Professor of Pharmacology, University College, London. He was elected to the Royal Society in 1931. He was the author of the classic textbook Applied Pharmacology.

British pharmacologist A. J. Clark conducted important research into the physiological reactions caused by drugs, with groundbreaking attention to the response to different doses of the same drug and the concentration of a drug and its effect at a receptor. Late in his career he publicly railed against popular but ineffective herbal and patent medicines, and pseudo-scientific advertising that sometimes fools even physicians into believing that such fraudulent remedies have some medicinal value.

Author of books:
Comparative Physiology of the Heart (1927)
The Mode of Action of Drugs on Cells (1933)
General Pharmacology (1937)
Patent Medicines (1938)