Off the shelf Reading Glasses are available in a variety of styles and colors for both men and women.  Although such glasses are generally considered safe and provide an excellent emergency back up, an individual prescription, as determined by an optometrist usually results in better visual correction and fewer headaches and visual discomfort. This is because off the shelf are one size fits all with a possible result that you may not be looking through the centre of the lenses.
This in turn may cause eye strain.  Also both lenses are of the same power and unsuitable for those whose right eye prescription differs from the left eye. In addition they do not correct astigmatism. Ready mades are useful spare glasses because they are relatively cheap. We always recommend that you have a pair of reading glasses custom made for you, to your prescription.