BACKGROUND This study described real-world palbociclib dosing patterns and associated impacts on treatment costs for HR+/HER2- metastatic breast cancer (mBC) in the US. METHODS Postmenopausal women with HR+/HER2- mBC who initiated palbociclib-based therapy (initiation date = index date) between 02/03/2015 (palbociclib approval) and 02/29/2016, and continuous quarterly activity 1 year before and 6 months after the index date, were identified in the Symphony Health Solutions database. Rates of 1) dose reduction (≥25 mg dose decrease/daily), and 2) reduction or interruption (>60 day gap in continuous drug supply) were assessed using Kaplan-Meier analyses. Drug wastage cost was estimated based on overlapping days between two prescription fills: the last original dose fill and the first reduced dose fill. RESULTS 1,242 patients initiated palbociclib-based therapy (mean age = 62.7 years, median follow-up = 8.7 months). During the 12-month post-index period, across the first four lines, dose reduction rates were 31.9-33.7% and dose reduction/interruption rates were 63.5-80.9%. A total of 411 (33.1%) patients changed dose, among whom 128 (31.1%) experienced prescription fill overlap (average = 11.1 days). Mean potential drug wastage cost among patients with fill overlap was $5,471. CONCLUSIONS Most patients receiving palbociclib experienced dose reduction or interruption early in treatment; the associated drug wastage may lead to considerable costs.