For most people ready made reading glasses are not as good as custom-made glasses, but they may be suitable as a spare or extra pair. Because they are made as 'one size fits all' the frame may not fit you and you may not be looking through the center of the lenses, which can cause eyestrain and headaches. Both lenses are also made the same, and therefore not ideal if one eye is different from the other. If you have astigmatism they do not correct it, making them less accurate. They are however, unlikely to do you any harm. It is still important to have regular eye examinations to make sure your eyes are healthy, particularly when you are over the age of 40. It is unlikely that you will make your eyes worse, but inevitably as you get older you will become more dependent on them and gradually need stronger lenses.
If you want to save money, it is ok for you to buy cheap reading glasses from online eyeglasses shop. However, before you buy it, you need to make sure that you have got your prescription for your eye doctor. Once your get your prescription, you can buy over the counter reading glasses, also, you can buy reading glasses. However, do not buy reading eyeglasses for cheap price and without notice the power of the lenses. As long as you wear the correct prescription lenses, it is safe for your eyes.
It depends. There are some good reading glasses that sold at low price especially online. They are cheap but they are not cheaply quality. If you buy such cheap reading glasses, and the eyeglasses are the right prescription for your eyes, it is OK if you wear them. They won't damages your eyes but also provide your vision aids. However, if you buy cheap reading glasses that is not the prescription that your eyes need, you may get eyes hurt such as eye pain, blurry vision or headaches etc. So, be careful if you want to buy cheap reading glasses online.
You mean over the counter reading glasses? It can be purchased in variety, drug, and discount stores without a prescription. They can be used instead of prescription reading glasses, but they are not an ideal substitute. These cheap reading glasses are simply magnifying glasses mounted in frames, they are not usually up to the quality of those ground in prescription laboratory, and the optical centers of the lenses are not likely to be in the precise position that gives you the best and most comfortable reading vision. The frames are likely to be one-size-fits-all. However, they may work for you. If so, they can come in handy if you like to have several pairs of reading glasses lying around in different places or if you tend to misplace your reading glasses.