OBJECTIVE: People who use both alcohol and combustible tobacco have an increased risk of developing cancer. Few interventions have been developed to inform people about the risks of co-use. This study developed and tested messages about the risks of alcohol and combustible tobacco co-use among adults. METHOD: In June to July 2021, we surveyed 1,300 U.S. adults who had used both alcohol and combustible tobacco products within the past 30 days. After reporting their awareness of diseases caused by tobacco and alcohol co-use, participants were randomly assigned to four between-subjects experiments that manipulated specific cancer health effects versus the word RESULTS: Awareness of health effects caused by alcohol and tobacco co-use ranged from moderately high for throat cancer (65.4%) to moderately low for colorectal cancer (23.1%). Messages about cancer health effects increased PME more than messages about noncancer health effects ( CONCLUSIONS: Messages about some cancer health effects of co-using alcohol and tobacco may be effective when communicating the harms of both drinking alcohol and using tobacco.