If you're not eating as much as usual during treatment, or if you're losing vitamins and minerals due to side effects like vomiting and diarrhea, you might consider taking a multivitamin. Vitamin D insufficiency is the most frequent vitamin deficit, Vitamin D maintains bone health, strengthens the immune system, and lowers weariness. You'll be at risk for bone density loss, especially if you're taking steroids. Before adding any vitamins or supplements to your diet, consult a qualified nutritionist and your oncologist.