IMPORTANCE: Extended-interval dosing of pembrolizumab (400 mg every 6 weeks) was approved by US Food and Drug Administration (FDA) in April 2020 as an alternative to standard-interval dosing (200 mg every 3 weeks). Extended-interval dosing may enhance access, alleviate patient and health system financial toxicity, and improve patient quality of life, particularly during the COVID-19 pandemic. Neither adoption nor effectiveness of extended interval in the US has been adequately described. OBJECTIVE: To describe adoption of extended-interval dosing of pembrolizumab since its FDA approval and to measure its preliminary real-world effectiveness compared with standard-interval dosing. DESIGN, SETTING, AND PARTICIPANTS: This was a retrospective cohort study that used data from the Veterans Health Administration (VHA), a US-based, nationwide single-payer health system. Participants were veterans who were prescribed single-agent pembrolizumab within the VHA between April 1, 2020, and July 1, 2021. Patients receiving combinations of pembrolizumab and cytotoxic chemotherapy or tyrosine kinase inhibitors were excluded. A subcohort of veterans with non-small cell lung cancer (NSCLC) was also identified using claims-based codes. EXPOSURES: Single-agent pembrolizumab at extended or standard intervals. MAIN OUTCOMES AND MEASURES: The number and proportion of single-agent pembrolizumab prescriptions that were extended compared with standard interval. Effectiveness was described in terms of time-to-treatment discontinuation (TTD) and extended- to standard-interval pembrolizumab prescriptions were compared using Cox proportional hazards regression. RESULTS: A total of 835 veterans (mean age [SD], 70.9 [8.7] years; 809 [96.9%] men) began single-agent pembrolizumab during the study period (all-diseases cohort), and of these, 234 (mean [SD] age, 71.6 [7.3] years; 225 [96.2%] men) had NSCLC (NSCLC cohort). Extended-interval adoption reached its steady state plateau of approximately 35% by January 2021; 65% of participants who began standard-interval single-agent pembrolizumab received only standard-interval dosing during the treatment course. In analysis consistent with the intention-to-treat principle, no differences in TTD were observed between standard- and extended-interval dosing in either the all-diseases cohort (HR, 1.00; 95% CI, 1.00-1.00) or the NSCLC cohort (HR, 1.00; 95% CI, 1.00-1.00). CONCLUSIONS AND RELEVANCE: This retrospective cohort study found that extended-interval dosing comprised a minority of single-agent pembrolizumab prescriptions despite the FDA approval and its potential health system and public health benefits. The findings support the TTD equivalence of standard- and extended-interval pembrolizumab across indications, complementing clinical pharmacology and single-arm clinical trial data in melanoma. This study provides further support for extended-interval pembrolizumab dosing.