Here is a common question I get asked at the pharmacy: “Should I take vitamin D, and what does it actually do?”

You may have heard that vitamin D helps bones, mood, immunity, even heart health. It can be hard to know which of these claims are true, but one thing is clear in a Canadian winter vitamin D can be an important supplement for many people.

What does vitamin D do?

There are different types of vitamin D. The most common over-the-counter supplement is vitamin D3, also called cholecalciferol.
Vitamin D helps your body absorb calcium from food. It also has effects on muscles and the immune system. Your skin can make vitamin D when the sun’s UVB rays hit it, but in a Canadian winter we do not get as much of this. You can also get some vitamin D from foods like fortified milk, fatty fish and eggs.
If you do not have enough vitamin D for a long time, bones can become weak. In children this can cause rickets. In adults it can cause soft bones and even bone pain.

Am I at risk of low vitamin D?

People at higher risk of low vitamin D include older adults, those who stay indoors or rarely go outside, individuals with darker skin, people with certain gut conditions that affect absorption, people who had weight loss surgery, and those who take certain medications that change how vitamin D is broken down in the body. In these groups, a supplement is often needed, especially in winter.

Where vitamin D clearly helps

For people who have true vitamin D deficiency, vitamin D helps correct low blood levels. Vitamin D also helps protect bones in older adults. Studies have shown that vitamin D taken with calcium can lower the risk of fractures. It is not a stand-alone treatment for osteoporosis, but it is an important part of bone care.

Can vitamin D prevent colds and flu?

There has been a lot of interest in vitamin D and immunity. Some studies have shown that vitamin D lowers the chance of acute respiratory infection, such as a cold or flu. The benefit was strongest in people who already had very low vitamin D and who took vitamin D regularly.
This means vitamin D may give some added protection for people with vitamin D deficiency, but the effect is modest.

What about mood, heart health and diabetes?

You may come across claims that vitamin D helps with depression, heart disease or diabetes. The results from studies in these areas have been mixed.
The bottom line is that correcting vitamin D deficiency may help overall health, but vitamin D by itself is not a recommended treatment for these long-term conditions.

How much should I take?

The right dose depends on your age, health, other medicines and blood level.
Many older adults need about 800 to 1,000 units of vitamin D per day to help with bone health and to reduce the risk of fractures and falls. Those with very low vitamin D levels or after weight loss surgery, may need higher doses under medical supervision.
For most healthy infants and children, the usual recommended dose is 400 units per day, often in a drop solution.

Using vitamin D safely

Usual doses of vitamin D are generally well tolerated, and serious side effects are rare. It is best to avoid very large “mega doses” unless they are prescribed and monitored.
Keep in mind that more is not always better. Most adults should not exceed 4,000 units per day. Very high long-term doses can lead to too much calcium in the blood. This can cause nausea, constipation, weakness and other problems.
Tell your pharmacist about the medicines you take, as vitamin D can interact with some drugs. If you have kidney disease, high calcium, or other complex conditions, talk to your doctor before starting a new supplement.

Take home message

In a Canadian winter, vitamin D is important, but it is not a cure all.
If you are wondering whether you or your family should take vitamin D this winter, talk to your pharmacist. We can help you decide if you need a supplement and what dose is right for you.

Originally published in East Gwillimbury’s Bulletin Magazine February 2026