An ad campaign for Camels from the mid 1940s. The Stanford Research into the Impact of Tobacco Advertising
site (where I found them) notes:
The none-too-subtle message was that if the doctor, with all of his expertise, chose to smoke a particular brand, then it must be safe. Unlike with celebrity and athlete endorsers, the doctors depicted were never specific individuals, because physicians who engaged in advertising would risk losing their license. (It was contrary to accepted medical ethics at the time for doctors to advertise.) Instead, the images always presented an idealized physician - wise, noble, and caring - who enthusiastically partook of the smoking habit. All of the "doctors" in these ads came out of central casting from among actors dressed up to look like doctors.