There are different forms of vitamin D, including ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Vitamin D is found in fish, eggs, and fortified milk. It's also made in the skin when exposed to sunlight. During periods of sunlight, vitamin D is stored in fat and then released when sunlight is not available.

Vitamin D supplements are commonly used to treat and prevent vitamin D deficiency. People who don't get enough sun and people who are 65 years or older are at risk for deficiency. People also use vitamin D for weak and brittle bones, heart disease, asthma, hay fever, and many other conditions, but there's no good scientific evidence to support many of these uses. There is also no strong evidence to support using vitamin D supplements for COVID-19. But it is important to maintain healthy levels of vitamin D. This can be done by taking 400-1000 IU of vitamin D daily or spending 15-30 minutes in the sun each day.