To predict potential public health impact of a candidate Modified Risk Tobacco Product (MRTP) requires pre-market information on how adult consumers actually use the product under close to real-world conditions. In this context, a Tobacco Heating System (THS) was assessed with regard to product adoption, use in conjunction with other tobacco products, and switching back to smoking cigarettes. Single group, four-week, consumer use studies were conducted between 2013 and 2015 in five countries (Germany, Italy, Japan, South Korea, and Switzerland). In total the analysis assessed how 2 809 adult smokers used THS based on participants' self-reported stick-by-stick consumption of THS and of conventional cigarettes (CC), using pre-defined product use categories (THS use, combined use, CC use). The results showed that: 1) After four weeks, between 10% and 37% of the study participants switched either exclusively or predominantly to THS Tobacco Sticks, with highest adoption rates observed in Japan and South Korea. 2) Across countries, between 27% and 39% of the study participants used both cigarettes and THS during the study period. This combined use decreased over time, with dual users either returning to CC use or, to a lesser extent, transitioning completely to THS use. 3) Across countries, the daily average total tobacco product consumption (THS Tobacco Sticks plus CC) was lower than the self-reported consumption of cigarettes prior to entering the study. The introduction of the candidate MRTP THS in various countries would result in adoption by some adult smokers who would otherwise continue to smoke cigarettes,and with varying level of adoption, as suggested by the studies' results in Europe and Asia.