Would it be fine to take a higher dosage of folic acid without a doctor's prescription?

YES: Higher doses of Folic Acid have been found to be improve sperm quality, reduce birth defects, lower risk of macular degeneration, and reduce homocysteine level associated with cardiac disease. It's also essential for the production of red blood cells and preventing anemia. It's generally safe because it's water soluble. So, toxicity risk is low. Nevertheless, it's best to take b-complex with it.