According to WebMD, weight gain is not listed as one of the possible side effects of taking folic acid. Usually, no side effects are noticed when taking the recommended dose of folic acid every day.
WebMD says that taking doses above the recommended 400 micrograms of folic acid sometimes causes side effects, such as rashes, sleep disorders, abdominal cramps, diarrhea and irritability. Doctors have some concerns that taking folic acid in doses over 800 to 1,200 micrograms increases the risk of heart attack in people who have heart problems. Other studies suggests that it also increases the risk of prostate and lung cancer when taken in high doses.