Trial of Vitamin D Supplementation in Asthma

Principal Inclusion Criteria: * Medical record diagnosis of asthma * Age ≥ 16 years and ≤ 80 years on day of first dose of IMP * If a woman of child-bearing potential, is sexually abstinent or has negative pregnancy test within 7 days of recruitment and agrees to use reliable form of contraception until she has completed the study * Able to give written informed consent to participate in the study Principal Exclusion Criteria: * Diagnosis of COPD * Known sarcoidosis, hyperparathyroidism, nephrolithiasis, active tuberculosis, vitamin D intolerance, liver failure, renal failure, terminal illness, lymphoma or other malignancy not in remission for ≥ 3 years * Any other condition that, in an investigator's judgement, might compromise patient safety or compliance, interfere with evaluation or preclude completion of the study * Taking benzothiadiazine derivative, cardiac glycoside, carbamazepine, phenobarbital, phenytoin or primidone * Taking dietary supplement containing vitamin D up to 2 months before first dose of IMP * Treatment with any investigational medical product or device up to 4 months before first dose of IMP * Breastfeeding, pregnant or planning a pregnancy * Baseline corrected serum calcium \> 2.65 mmol/L * Baseline serum creatinine \> 125 micromol/L * Smoking history \>15 pack-years