This study is for people with multiple myeloma, a type of cancer that affects blood cells. Participants will be split into two groups. One group will receive Vitamin D after a stem cell transplant, while the other group will not. Both groups will be treated with lenalidomide, a drug used to treat multiple myeloma. The study will see if Vitamin D helps the immune system and improves survival rates.

• The study lasts for about 3 years, with regular check-ups and tests.
• Participants must be 18 years or older and have had multiple myeloma needing treatment.
• Some people with other health issues might not be able to join, so a health check is necessary.

To join, participants must follow certain rules, like taking medications safely and, if women, avoiding pregnancy during the study. Participants will need to take Vitamin D or a placebo and attend regular doctor visits for 3 years. It's essential to understand the requirements, risks, and benefits before deciding to join the study.