I just found this on a website:
"The question that remains is: Can you overdose on folic acid? Can taking a high dose of folic acid endanger your health? Well, first of all, the folic acid maximum dose has been established to be 1000 micrograms, though there have been no significant reports of folic acid overdose.
While this is true, excess folic acid (in the milligram levels) has been seen to mask symptoms of vitamin B12 deficiency. Also, excessive amounts have been found to prevent action of some medications such as anti-epilepsy. These are some of the reasons why the upper limit of 1000 micrograms (1 milligram) has been set. However, if you accidentally take too many supplements in a day, you will not likely absorb higher than 1000 milligrams."