Hugh Desiax Riordan M.D. (May 7, 1932 – January 7, 2005) was an American psychiatrist and researcher. He was the co-founder and, until his death, the director of the Olive W. Garvey Center for the Improvement of Human Functioning (now called the ). He was best known for his belief that nutrition and vitamins are effective treatments for diseases such as cancer. This approach to patient care is known as orthomolecular medicine and is dismissed by the mainstream medical community.