Cannabis is commonly smoked and often co-used with tobacco, yet evidence on interventions to reduce smoking harms for people who use cannabis is limited. We systematically reviewed evidence on the efficacy and effectiveness of health education interventions (aiming to increase health literacy, knowledge, or motivation) in reducing cannabis smoking and tobacco smoking among people using cannabis. We searched five databases (inception–February 2025) for quantitative evaluations of health education interventions targeting cannabis use and reporting cannabis and/or tobacco smoking among people who use cannabis. Evidence from 32 studies was synthesized narratively, with risk of bias assessed using design-appropriate tools. Samples ranged from 15 to 10 781 participants, predominantly adolescents and young adults. Half evaluated interventions to prevent uptake, mostly school-based programmes, which showed mixed impact in reducing cannabis smoking. Half promoted quitting/reduction and provided some evidence that motivational interventions reduced cannabis smoking. Tobacco outcomes were rarely assessed (k = 7) and largely null; risk of bias was high throughout. Overall, school-based preventive programmes showed mixed effects, while motivational interventions more consistently reduced cannabis but not tobacco smoking. Even modest effects may benefit public health, highlighting the need to clarify effective components, delivery settings, and to standardize measures for routes of administration and co-use.