Although rare, there are a number of side effects of folic acid that you should be aware of. There are a number of milder side effects where you may be able to continue taking folic acid supplements but there are more serious side affects as well where you are recommended to immediately stop using folic acid. In all circumstances we recommend speaking to your doctor about the side effects you are experiencing so that he can further assist you.
The more serious side affects are really allergic reactions to folic acid and include difficulty breathing, closing of the throat, swelling of your lips, tongue or face, or hives. (http://www.drugs.com/Folic_Acid/index.html) If you experience any of these symptoms please see your doctor and stop using folic acid.
Some milder side effects you may experience are nausea, loss of appetite, bloated abdomen, flatulence, bitter taste, insomnia or difficulty concentrating. In the case of these side effects it is still recommended that you visit your doctor but you may not need to stop your supplementation of folic acid, consult with your doctor first.
http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202250.html also mentions "fever, general weakness or discomfort; reddened skin; shortness of breath; skin rash or itching; tightness in chest; trouble breathing" and wheezing amongst the possible side effects which may occur when using folic acid.
According to the University of Maryland, "side effects from folic acid are rare. Very high doses (above 15,000 mcg) can cause stomach problems, sleep problems, skin reactions and seizures."
The Encyclopedia of alternative medicine (http://health.enotes.com/alternative-medicine-encyclopedia/folic-acid) says the following: "Folic acid is generally considered safe at levels of 5,000mcg or less. Side effects are uncommon. However, large doses may cause nausea, decreased appetite, bloating, gas, decreased ability to concentrate, and insomnia. Large doses may also decrease the effects of phenytoin (Dilantin), a seizure medication.
It is also recommended if you are taking a folic acid supplement that you also take a supplement including the other B Vitamins, especially Vitamin B12, as Folic acid can mask the symptoms of Vitamin B12 deficiency and lead to serious damage due to this deficiency rather than to the use of folic acid.
Side effects from folic acid are rare and are usually caused from a high dose of folic acid, where most people tend to be deficient. However if you experience any of the side effects mentioned above, especially shortness of breath, wheezing or swelling then it is recommended that you see your doctor as soon as possible. For milder side effects you may be able to continue taking folic acid supplements but this should be discussed with your doctor.