BACKGROUND: Complementary and Alternative Medicine (CAM) is widely used among cancer patients worldwide. This prospective observational study aimed to show the effect of CAM use on chemotherapy delivery in Thai patients. METHODS: During March 2014 to February 2015, the patients with breast, lung or colorectal cancer receiving first cycle chemotherapy at King Chulalongkorn Memorial Hospital were enrolled. The correlation between CAM using and chemotherapy schedule delay and dose reduction, dose intensity, quality of life and adverse event rates were analyzed. RESULTS: There were 80 (44.20%) patients using CAM among 181 enrolled patients.  Seventy six CAM users and 97 non-CAM users receiving 2nd cycle of chemotherapy were included for primary analysis. The chemotherapy schedules were delayed and/or reduced in 40 (52.6%) and 48 (49.5%) in CAM users and non-CAM users, respectively, p =0.681. The mean relative dose intensity (RDI) were 92.4% and 94.1% in CAM and non-CAM users, respectively, p=0.244. However, there were significantly more CAM users receiving chemotherapy less than 90% RDI (34.8% vs 19.8%, p=0.033). As compared to first cycle, at third cycle, the mean QOL score changes were -4.63 (95% CI -2.49-9.27) and -8.02 (-2.36- 9.142) in CAM user and non-CAM user, respectively (p=0.255). There were significantly higher rates of grade 3 or 4 anemia (5.1% vs 0%, p=0.024), and grade 2 malaise (19.0% vs 5.1%, p=0.004) in CAM users. CONCLUSIONS: There were similar overall rates of chemotherapy schedule delay and dose reduction between CAM- and non-CAM users. However, there were less CAM-users achieving 90% chemotherapy RDI.