Objective: Medication possession ratios (MPRs) can be misleading as an indicator of adherence, especially where several medications for the same indication are used concurrently. We investigated whether concurrent utilization of controller asthma medications in patients switching therapies differs between controller medication classes and whether it may lead to significant biases in observed patterns of medication possession and inappropriate claims for long-term patient adherence. Methods: We used MPRs as a proxy measure of adherence to asthma controller medications. Asthma controller utilization patterns were investigated using the Pharmetrics Managed Care database for the period April 2003 to March 2006. Asthma patients who were exclusively on two of three major controller groups were identified: inhaled corticosteroids (ICSs), fluticasone/salmeterol (FS), and leukotriene modifiers (LTMs). MPRs were calculated for days of concurrent possession (DCPs) between last and first prescription recorded, as well as the proportion of days that were covered by concurrent controller possession (CCPs). Results: Overall for the study population (n=7260), 1328 patients had prescriptions for LTMs and ICSs; 4871 for LTMs and FS; and 1061 for ICSs and FS. The mean CCPs were highest for persons reporting prescriptions for both LTMs and FS (0.224). Lowest CCP was for ICS and FS (0.023). DCP over the 3-year observation period was highest for persons with LTMs and FS (0.152 or 166 days) and lowest for those with ICSs and FS (0.009 or 10 days). Conclusion: The analysis points to a significant number of asthma patients using LTMs concurrently with another controller medication. This occurs despite the lack of clinical evidential support for concurrent utilization over an extended period. Estimates of MPRs for controller therapies based on prescription counts need to be corrected for concurrent medication usage to avoid overstating therapy adherence.