BACKGROUND: The use of high-dose vitamin D supplementation has increased in recent years. However, relatively little is known about the safety of long-term high doses. AIMS: To investigate the safety of a monthly high-dose of vitamin D3 supplementation taken for up to 4 years. METHODS: Data were collected in a randomized, double blind, placebo-controlled trial of 5108 adults aged 50-84 years old from Auckland, New Zealand. Participants were given monthly doses of 100,000 IU vitamin D3 or placebo, for a median of 3.3 years (range 2.5-4.2 years). They answered an open-ended question in a monthly questionnaire about any adverse events they attributed to the study capsules, which were coded blindly. Incidence rates per person months were calculated for categories of adverse events. Cox regression model used to calculate hazard ratio of time to first adverse-event. RESULTS: In total, 419 (16.5%) participants taking vitamin D and 399 (15.8%) taking placebo reported ≥1 adverse event. Compared to placebo, the hazard ratio (HR) of reporting first adverse event in the vitamin D group was 1.03 (95% CI: 0.90, 1.18; p = 0.63). Despite a slightly higher incidence of recurrent adverse events in vitamin D arm, the incidence rate ratio (1.17) was not significantly higher in vitamin D (95% CI: 0.97, 1.41; p = 0.10). All regression results were adjusted for age, sex, and ethnicity. There was no difference between study arms in terms of participants' allocation perception (p = 0.52). CONCLUSION: Monthly supplementation of 100,000 IU vitamin D3 for a median of 3.3 years did not affect participant-reported adverse events. TRIAL REGISTRATION: clinicaltrials.gov Identifier: ACTRN12611000402943; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?ACTRN=ACTRN12611000402943.