Importance: While rates of cigarette use are declining, more US adults are using cannabis. Perceptions of safety are important drivers of substance use and public policy; however, little is known about the comparative views of US adults on tobacco and cannabis safety.

Question: Is daily smoking of cannabis or exposure to secondhand smoke believed to be safer than tobacco smoking or exposure, and have perceptions changed over time?

Findings: In this survey study of 5035 US adults, daily cannabis smoking or smoke exposure was perceived to be safer than tobacco. Over time, views increasingly favored the safety of cannabis vs tobacco smoke.

Meaning: These findings suggest that public health efforts may be necessary to educate the public on potential risks and curb the increasing social acceptance of cannabis smoke exposure, similar to past education about secondhand tobacco smoke.