What is the treatment for refractive error that is best and has fewest side effects?

Optical aids: Refractive error simply is an eye with an optical difference from "normal" requiring eyeglasses or contacts to correct. All methods are good, there are no side effects. If the optics are not perfect the vision may not be perfect but this will not harm the eye. It is always important to see your best so get your glasses. Later in life you could have lasik for a non-eyeglass fix.

Depends on the cause: There are several ways to treat vision problems related to refractive errors (need for prescription correction). The easiest is to wear glasses. Contact lenses can decrease your need for glasses, but must be cared for regularly in order to prevent infections. Laser vision correction and lens replacement surgery are alternatives that can provide lasting glasses-free vision but are costlier.