(1) Background: The prevalence of complementary and alternative methods (CAM) use among oncological patients has been rising constantly over the last few decades and a variety of both pharmacological and non-pharmacological methods have been developed. Many advertisements promise to relieve side effects of chemotherapy or even to cure the disease, thus encouraging patients to use CAM; (2) Methods: The objective of the study was to determine which patients’ characteristics are associated with the use of complementary medicine during cancer treatment, their pattern of use, and if it has any association with its safety profile. This survey-based prospective multicenter study of 316 patients examined the use of complementary medicine among patients undergoing chemotherapy treatment in cancer centers in Poland between 2017 and 2019; (3) Results: The Chi2 analysis showed that patients’ opinion regarding the safety of unconventional methods is related to the use of CAM (p
= 0.00147). Moreover, patients’ thinking that alternative medicine can replace traditional therapy was correlated with his/her education (p
= 0.01198). Moreover, we performed univariate and multivariate analysis to determine factors associated with CAM use including sociodemographic and clinical characteristics. Finally, we conducted survival analysis of patients undergoing chemotherapy treatment with 42 months of follow-up time of our prospective study. Using Kaplan–Meier curves and log-rank analysis, we found no statistical difference in overall survival between the groups that used and did not use any form of CAM (p
= 0.211); (4) Conclusions: CAM use is common among patients undergoing chemotherapy treatment and should be considered by medical teams as some agents may interact with chemotherapy drugs and affect their efficacy or cause adverse effects.
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