Leo Zeff was a psychologist and pioneer in the psychedelic therapy movement. As a young man, Zeff was a Lieutenant Colonel in the U.S. Army. Later, after obtaining his PhD, he opened a private practice specializing in Jungian psychology. Eventually he discovered how effective psychedelics could be in helping heal his patients. When psychedelics were outlawed, Zeff went underground.

Using the pseudonym "Adam Fisher", Zeff is mentioned by Alexander and Ann Shulgin in their book PIHKAL. In 1977, at a point in his life when he was about to close down his Oakland-based practice, Zeff was turned on to MDMA by Alexander Shulgin. After trying the drug, he abandoned his retirement plans and traveled across the country introducing MDMA to other therapists. It has been speculated by one of his friends that Zeff was responsible for turning on around 4,000 other therapists to MDMA. Zeff is responsible for coining the term "Adam" for MDMA, as he felt that it stripped away the ego's defense mechanisms and returned the user to a primordial state of innocence.

Under the pseudonym "Jacob", Zeff's work is detailed in Myron J. Stolaroff's book The Secret Chief. Published in 1997, the book is based on a series of interviews that Myron and Jean Stolaroff did with Zeff in 1981. It describes the materials and methods that Zeff found to be of value, and provides numerous first-person accounts attesting to the value of psychedelic therapy. A second edition, published in 2004, reveals Zeff's real name for the first time, as his family felt that enough time had passed that there was no longer any risk to his past patients and associates due to drug prohibition laws.

By all accounts, Leo Zeff was a remarkable person, beloved by his friends, colleagues, and clients.