The investigators hypothesize that vitamin D levels can be improved in hematopoietic stem cell transplantation (HSCT) recipients not responding or tolerating standard enteral supplementation options by using vitamin D oral thin film (OTF) administration.

The purpose of this study is to investigate efficacy of oral thin film (OTF) cholecalciferol (vitamin D3) replacement in hematopoietic stem cell transplantation (HSCT) recipients who failed to achieve or sustain adequate vitamin D levels after vitamin D supplementation using current standard of care, or those unable to take/tolerate currently available enteral vitamin D formulations. With oral thin film (OTF) cholecalciferol, investigators hope to significantly improve the ease of administration, restoring compliance and facilitating therapeutic vitamin D levels without relying on the gut for absorption.

Inclusion Criteria:

• HSCT recipients of any age with vitamin D levels ≤35 ng/mL, or unable to tolerate, or refractory to enteral supplementation formulations of Vitamin D will be offered Vitamin D OTF.

Exclusion Criteria:

• Subjects with 25OHD level >60ng/mL. Subjects can be re-screened multiple times and enrolled later if they meet study criteria.
• Subjects with clinically significant and uncontrolled hypercalcemia as deemed by treating physician.