Both the AMA and individual doctors sided with big tobacco for decades after the deleterious effects of smoking were proven. Medical historians have tracked this relationship in great detail, examining internal documents from tobacco companies and their legal counsel and public relations advisers. The overarching theme of big tobacco's efforts was to keep alive the appearance of a "debate" or "controversy" of the health effects of cigarette smoking.
The first research to make a statistical correlation between cancer and smoking was published in 1930 in Cologne, Germany. In 1938, Dr. Raymond Pearl of Johns Hopkins University reported that smokers do not live as long as non-smokers. The tobacco industry dismissed these early findings as anecdotal -- but at the same time recruited doctors to endorse cigarettes.