Research on the use of folate and folic acid supplements for specific conditions shows:

Birth defects. Folic acid supplements can prevent birth defects that affect the brain and spinal cord. Taking a daily prenatal vitamin can help make sure you get enough folic acid. If you plan to get pregnant, take the supplement at least one month before trying to get pregnant and until at least 12 weeks into pregnancy.
Folate deficiency. Take a folic acid supplement if you have trouble absorbing or getting enough folate in your diet. Many countries add folic acid to foods, such as cereals and pasta. So most people rarely need to take folic acid supplements.
Heart and blood vessel disease and stroke. Folic acid works with vitamins B-6 and B-12 to control high levels of homocysteine in the blood. Too-high homocysteine levels might raise your risk of heart and blood vessel conditions, also called cardiovascular disease. Taking folic acid supplements also may lower your stroke risk.
Cancer. Some research suggests that folate might lower the risk of various cancers.
Depression. Some evidence suggests that folic acid might be a helpful supplement in treating depression.
Dementia. There isn't enough evidence to support taking folic acid to prevent dementia.
Autism spectrum disorder. Some research suggests that taking folic acid when pregnant may help lower the risk of autism spectrum disorder in the child.