This information is for people who have had a stroke or mini-stroke. It tells you about vitamin B supplements, a treatment used for stroke prevention. It is based on the best and most up-to-date research.
Do they work?
Probably not. Combinations of B vitamins don't seem to work, and the B vitamin folic acid on its own doesn't seem to help either.
What are they?
Vitamins are nutrients that your body needs to work properly. There are several types of vitamin B. In some of the research on preventing a stroke, people took a pill containing three B vitamins. In other research, people took only folic acid. The three vitamins that have been looked at in clinical trials were:
Vitamin B6. This is found in several foods, including pork, chicken, bread, soya beans, peanuts, milk, and potatoes 
Vitamin B12. This is found mainly in animal products, such as meat, dairy products, and eggs. There's also some vitamin B12 in yeast extracts, such as Marmite 
Folic acid, also called vitamin B9. The natural form of this vitamin, found in foods, is called folate. Small amounts of folate are found in lots of foods. Good sources are broccoli, Brussels sprouts, peas, chickpeas, and brown rice. 
The supplement used in some studies contained 50 milligrams (mg) of vitamin B6, 1 mg of vitamin B12, and 2.5 mg of folic acid.
How can they help?
It is unlikely that they can help. A summary of 12 studies found that that taking B vitamins worked no better than taking a dummy treatment (a placebo) at preventing strokes.
One study looked at more than 5,000 women.  They were followed for seven years, and took either a combination of B vitamins or a dummy treatment (a placebo).
The researchers looked at the number of people who had a stroke. There wasn't much difference between people taking B vitamins and people taking a placebo. About 3 in 100 people had a stroke over the seven years, whether they took B vitamins or a placebo.
Taking folic acid (vitamin B9) on its own is unlikely to lower your chances of having another stroke. A summary of eight studies found that taking folic acid didn't seem to work any better than taking a placebo. 
How do they work?
High levels of a chemical called homocysteine in your blood may play a part in increasing your risk of a heart attack or stroke. Vitamin B supplements help reduce the levels of this chemical. So researchers thought B vitamins might also protect against strokes.
The research on vitamin B doesn't seem to show any benefits.
Can they be harmful?
The research doesn't give much information about side effects.
Taking very high doses of vitamin B6 (more than 200 milligrams a day) can lead to nerve problems and a loss of feeling in your arms and legs.  Most of the time, these side effects go away when you stop taking these vitamin supplements. The dose that causes this side effect is much higher than the dose used in the research, and higher than the amount found in most vitamin pills you can buy.