no hun, its fine to take without it being prescribed, they say to take it as soon as you find out you are pregnant, and the boxes sold in shops/boots are the correct stregnth etc. Or what I have done is take a prenatal vitamin (Tommys) that includes folic acid along side all other recommended vitamins including omega 3 and DHA I think its called. then you know you are getting the correct dose of all needed vitamins every day.
Just remember to only take recommended dose on the box.