Leffingwell authored many books bringing light to the cruel abuses of animal experimentation and calling for regulation.

At the same time, he sought middle ground between the anti-vivisection societies, which called for the abolition of all experimentation and those who rejected any restraints.

Leffingwell also was concerned with meat safety, believing that lax regulations, in particular allowing cancerous animals into the food chain, were responsible for increases in the incidence of cancer.

[2] From 1895 to 1906, Leffingwell corresponded with Sarah James Eddy on the topic of animal welfare and vivisection.

[3] This biographical article related to a physician in the United States is a stub.