The study null hypothesis is that vitamin D supplementation will not influence time to upper respiratory tract infection or time to severe asthma exacerbation in adult and adolescent patients with 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