Background Vitamin D plays a role in host defence against infection. Vitamin D deficiency has been associated with an increased risk of respiratory tract infections in children and adults. This study aimed to examine whether vitamin D supplementation is associated with a lower pneumonia risk in adults.

Methods Three independent case–control studies were performed including a total of 33 726 cases with pneumonia in different settings with respect to hospitalisation status and a total of 105 243 controls. Cases and controls were matched by year of birth, gender and index date. The major outcome measure was exposure to vitamin D supplementation at the time of pneumonia diagnosis. Conditional logistic regression was used to compute ORs for the association between vitamin D supplementation and occurrence of pneumonia.

Results Vitamin D supplementation was not associated with a lower risk of pneumonia. In studies 1 and 2, adjustment for confounding resulted in non-significant ORs of 1.814 (95% CI 0.865 to 3.803) and 1.007 (95% CI 0.888 to 1.142), respectively. In study 3, after adjustment for confounding, the risk of pneumonia remained significantly higher among vitamin D users (OR 1.496, 95% CI 1.208 to 1.853). Additional analyses showed significant modification of the association through co-use of corticosteroids and drugs that affect bone mineralisation. For patients using these drugs, ORs below one were found combined with higher ORs for patients not using these drugs.

Conclusions This study showed no preventive association between vitamin D supplementation and the risk of pneumonia in adults.