Co-users of cannabis and tobacco frequently use cannabis, then tobacco cigarettes, in a sequential pattern within an occasion, that is, they "chase" smoked cannabis with a tobacco cigarette. The objective of this placebo-controlled, double-blind, within-subjects human laboratory study was to gather preliminary data on how smoking active versus placebo cannabis impacts tobacco cigarette smoking behavior, craving, and subjective effects. Adult daily cannabis and tobacco co-users (N = 9) were randomly assigned to two experimental visit orders (i.e., active cannabis (5.2% THC) first visit and placebo cannabis second visit, or vice versa). Participants smoked one cannabis cigarette, and approximately 30 min later were given a 5-min ad libitum period to smoke one of their own brand of tobacco cigarette. As expected, boost in plasma THC levels and cannabis-related subjective effects differed between active and placebo cannabis conditions. Tobacco cigarette puff topography measures and tobacco craving did not differ between cannabis conditions, but there appeared to be between-participants heterogeneity in cumulative total puff volume. After smoking active versus placebo cannabis, the changes in subjective effects of tobacco smoking after adjusting for pretobacco smoking levels were not significant. Results do not support the notion that immediate effects of smoked cannabis change the behavior of tobacco smoking. The strong overlap between cannabis and tobacco smoking may not be explained by primarily pharmacological factors, but may be driven by more nuanced and complex mechanisms involving pharmacological processes as well as learning factors. (PsycInfo Database Record (c) 2021 APA, all rights reserved).