They can cause damage to the eyes if they are too tight, worn for too long at a time, slept in, etc.
Jane Lindell Hughes, MD, FACS
Simply using regular prescription contact lenses, particularly if they are soft contacts which is what most people wear, increases your risk of serious infections such as corneal ulcers. If they are worn overnight, the risk goes up even higher. Extended wear contacts? The risk goes up even more.
But patients who need to see are willing to accept some of these risks so that they can function.
Someone who just wants their eye color to be different is taking a significant chance of harm to their eyes for a cosmetic reason. This risk can be much higher for people who get these colored contacts from novelty stores or places that don't take the time to ensure a proper fit.
Bottom line: colored contact lenses are not advisable, especially for younger people. If an adult still insists on getting these, they should at least see an eye doctor such as an optometrist to ensure that they are getting a health product and that it is properly fitted and maintained and subject to proper followup exams.