Whether intentional or not, mixing drugs is never safe because the effects from combining drugs may be stronger and more unpredictable than one drug alone, and even deadly.
Links
[1] https://www.cdc.gov/stopoverdose/polysubstance-use/index.html
[2] http://www.rethinkpot.org/tags/polysubstance-abuse
[3] http://www.rethinkpot.org/tags/ccd