Cancer pain is common, occurring in up to 60% of patients and opioid conversion may be required for effective pain management. Conversion from one opioid to another can be problematic due to differences in equianalgesic ratios found in established resources. This study explores the implications of using various published equianalgesic ratios when converting to a common opioid unit. This secondary analysis includes 105 advanced cancer patients who reported use of transdermal fentanyl, long-acting oxycodone, or oral methadone. Common clinically used equianalgesic ratios were identified and utilized to calculate a parenteral morphine equivalent for each of the selected agents. When the equianalgesic ratios were applied to each drug, there were substantial differences in the calculated morphine equivalent for transdermal fentanyl (2-fold difference) and methadone (100-fold difference). The calculated difference for oxycodone was lower, with a 1.5-fold difference. This study demonstrates large variability in opioid conversions based on the use of common equianalgesic ratios for transdermal fentanyl, long-acting oxycodone, and methadone. These findings have important clinical and research implications. First, this study substantiates the use of these ratios as only guidelines for treatment. Second, it supports the need for well-designed, rigorous studies to evaluate opioid conversions. Third, this study demonstrates the need for a standard reporting system of opioid equianalgesic ratios employed in clinical trials.