High Dose Vitamin D Plus Multivitamin in the Prevention of Cluster Headache

This study intends to investigate the use of high-dose Vitamin D3 plus a multivitamin in the prevention of cluster headache attacks. Participants can be enrolled anywhere in the United States that has access to one of our participating labs (for blood work - anticipated to be available in most of the USA). The study may include: * Screening: Participants may be interviewed, examined, fill out surveys, and get blood testing * Week 1: baseline period (no added medications - to establish a baseline) * Weeks 2-4: double-blinded experimental period - participants receive either 1) high-dose Vitamin D3 + multivitamin, or 2) placebo + multivitamin. Participants also fill out a survey and have blood testing. * Weeks 5-7: open-label period - ALL participants receive high-dose Vitamin D3 + multivitamin. Participants also fill out a survey and may have blood testing. Specifically, our primary outcome is the change from baseline to experimental weeks 1-3 in the frequency of cluster headache attacks between placebo and high-dose vitamin D. Specific primary and secondary outcomes are listed below.