Published online ahead of print in the journal Nicotine & Tobacco Research on June 12, 2021, this study examined 3 cohorts: tobacco-only users, cannabis-only users, and users of both tobacco and cannabis.
Tobacco-only and co-users of cannabis and tobacco had higher levels of exhaled carbon monoxide and other toxins when compared to cannabis-only users. Tobacco-only users had higher levels of 2-hydroxypropylmercapturic acid than co-users or cannabis-only users.
"Co-users and ET [exclusively tobacco] smokers demonstrated comparable levels of biomarkers of exposure to harmful constituents despite smoking similar amounts of tobacco. ECa [exclusively cannabis] smokers demonstrated lower levels of toxicant exposure for most biomarkers."
The study abstract and link to paper (paywall) is here.