This information is for people who have high blood pressure. It tells you about taking calcium supplements. It is based on the best and most up-to-date research.
Do they work?
We're not certain. There's some evidence that calcium supplements might help to lower your blood pressure a tiny bit. But we need to see the results of more research before we know for certain.
Having high blood pressure increases your chances of having a heart attack or a stroke. We don't know if taking extra calcium will reduce this risk. There's no good research to tell us.
What are they?
Calcium supplements are tablets that you can buy over the counter from a pharmacy or from health food shops. The label might say something like calcium carbonate or calcium citrate.
Calcium is best known as the building block for bones and teeth. It also keeps our hearts pumping and nerves working properly.
How can they help?
Taking calcium supplements (500 milligrams to 2,000 milligrams a day) might help to reduce your blood pressure a tiny bit. 
A few studies show that taking 500 milligrams to 2,000 milligrams of calcium a day lowered the top number of the blood pressure reading (the systolic pressure) by about 1.4 points on average, and the bottom number (diastolic pressure) by about 0.8 points.  (To find out more about the numbers, see Understanding your blood pressure reading.)
If you have high blood pressure, you are at risk of having a heart attack or a stroke and getting kidney disease. Studies haven't shown whether these risks are cut in people who take calcium tablets.
How do they work?
We're not sure how calcium might work in the body to lower blood pressure. But studies have found that people whose diet is low in calcium tend to have higher blood pressure than people whose diets have a lot of calcium.
It's possible that if you don't have enough calcium your blood vessels don't relax properly to let blood through easily. This makes it more difficult for blood to flow through them, so your blood pressure goes up.
Calcium may also help your body get rid of salt. Reducing the amount of salt in your body may lower your blood pressure. 
Can they be harmful?
Some people who take calcium supplements get mild stomach pain and constipation.
How good is the research on calcium supplements?
There hasn't been much research looking at whether calcium supplements can lower blood pressure. We found one summary of the research that weighed up the results of 42 studies. 
The studies looked at 4,560 middle aged men and women in total. Some of the people had high blood pressure and some didn't. The studies found that taking calcium supplements lowered blood pressure by a small amount. But most of the studies were very small. This makes it hard to say whether their results are reliable.