Glasses or contact lenses have long term bad effects on eyesight and it may make eyesight worse than before.
Whenever you wear new pairs of eyeglasses you would feel a slight discomfort adjusting to it. You may feel dizziness and problems with depth perception and this may take up to 2 weeks before your eyes adjust properly to the new grade lenses.
While wearing eyeglasses can improve eyesight instantly, it's not the best solution that address the root cause of the vision problem. Most people experienced degrading quality of vision over time while wearing eyeglasses.
As years went by, you would need a stronger prescription to "improve" your eyesight. Most nearsighted people go through myopic progression as time goes by.
One reason why this happens is because the eye muscles are so used to contract with the eyeglasses that it changes shape drastically which in turn worsens vision because the eyeball focus has changed drastically.
Using your glasses for nearsightedness would be bad for reading up close. The focus required by the eyes to look at a far distance is different from the focus from a 30 cm distance.
The eyes will have more strain while reading because the focus of eyeglass lenses is too much and should be used for distances beyond 6 meters only.
Wearing glasses also affects the size of our eyes. There are lot of research done on the effects of glasses on changing the shape of the eyeball. It makes the eyeball longer and this muscle may have a hard time going back to normal size if it's in constant strain