An interim analysis demonstrated no benefit of high dose vitamin D when compared to standard dose vitamin D in the reduction or prevention of arthralgia

This study is being done to look at the difference, if there is a difference between two different doses of Vitamin D and the reduction of joint/muscle pain (arthralgia)that is caused by taking anti-estrogen medications (aromatase inhibitors) by breast cancer patients. The investigators hope to learn if taking a higher dose of Vitamin D is a good way to prevent aromatase inhibitor arthralgia (AIA).

Treatments with anti-estrogen agents for hormone receptor positive breast cancer is the most efficacious of systemic therapies, with aromatase inhibitors (AI's) being considered the most active anti-estrogen therapy in early stage breast cancer. But, use of these treatments has been shown to cause musculoskeletal (joint/muscle) side effects that sometimes cause patients to discontinue the use of them. Also, Vitamin D deficiency is a well know cause of a wide array of musculoskeletal issues. There is evidence that Vitamin D supplementation may help prevent arthralgia while on AI's. Therefore, the investigators want to see if giving a higher dose of Vitamin D could decrease the incidence of AIA as compared to a standard dose of Vitamin D. The investigators believe that this could possibly result in patients continued treatment with AI therapy for hormone receptor positive breast cancer.

Inclusion Criteria:

• All participants must be female and at least 21 years of age
• Signed informed consent
• Patients must have had histologically confirmed stage I-III breast carcinoma that is positive for Estrogen Receptor (ER) and/or Progesterone Receptor (PR).
• Post-menopausal
• Beginning adjuvant aromatase inhibitor therapy, with no previous use within the last 6 weeks
• Bisphosphonates are allowed at the treating investigator¡¦s discretion
• Performance status (WHO/ECOG scale) 0-2.

Exclusion Criteria:

• History of kidney stones
• Hypercalcemia at baseline, defined as any corrected calcium greater than the laboratory's normal parameters
• History of either symptomatic hypercalcemia or hyperparathyroidism, at the treating investigator's discretion
• Baseline Vitamin D level greater than 50 ng/mL
• Inability or unwillingness to comply with, or follow study procedures.
• Currently taking Phenytoin or phenobarbital -7 Currently taking cholestyramine or orlistat
• Malabsorption syndrome, such as Crohn's disease

Prohibited Therapies: Patients may not take additional Calcium and Vitamin D aside from the study medications. Patients who are on cholestyramine or orlistat will not be allowed on the trial. Also, patients who are taking phenytoin or phenobarbital are not allowed on the trial either because of interaction between Vitamin D and anti-epileptic medications.

56 weeks. Adverse events experienced by participants were collected and reported from initiation of study medication, throughout the study (52 weeks treatment), and within 30 days of the last dose of study medication.