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Substance use spans a continuum from occasional use to hazardous use and clinically defined substance use disorders, and it can shape outcomes across the cancer care continuum. This structured narrative review synthesises evidence on how alcohol, tobacco, cannabis and opioids relate to cancer outcomes relevant to supportive care, including diagnostic timing, adherence, toxicity, symptom burden and survival. Methods We searched PubMed, Scopus and Web of Science for peer-reviewed studies published between January 2010 and April 2025 and narratively synthesised findings given heterogeneity in study designs and exposure definitions. Results Heavy or hazardous alcohol use and continued tobacco use are well-established carcinogenic exposures and are associated with delayed diagnosis, poorer treatment adherence, greater toxicity and inferior survival in observational studies. Evidence for cannabis and opioids is heterogeneous and predominantly observational; interpretation is limited by variable exposure measurement, including product composition and route of administration and dose, as well as polysubstance use, confounding and reverse causation. For opioids specifically, it is essential to distinguish analgesic use for cancer pain, physiological dependence and opioid use disorder. Conclusions To improve patient-centred outcomes, supportive oncology should embed routine screening, brief interventions and clear referral pathways, while implementing integrated oncology-addiction care models tailored to symptom burden and patient-reported outcomes. Future research should prioritise standardised exposure definitions, prospective longitudinal designs and evaluation of integrated care models in supportive oncology.