This is probably one of the most commonly asked questions regarding contact lenses. Those that do not wear them regularly see contacts as a mere commodity.
In the United States contact lenses are labeled as a medical device, and are therefor regulated by the food and drug administration. They are not medical devices because they provide a vision correction, but rather because they rest directly on the eye and become part of the entire ocular system.
When used properly, contact lenses are amazingly safe. However, when used improperly they pose serious health risks. Contacts have been known to cause serious infections, the most extreme of which can cause permanent loss of vision.
Therefor, the answer to the question is that you must be fit properly for any contact lens by a licensed eye care provider. Even if they are strictly for cosmetics, such as Halloween contact lenses.
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Before you get discouraged, you should know that it is incredibly easy to be fit with lenses. Simply visit your local eye care provider and let them know what you are interested in. They will give you a thorough eye exam to ensure you can safely wear lenses. This probably will take less than 10 minutes. And hopefully your exam will prove that your eyes are healthy and free of glaucoma or other silent thieves of sight.
Next, your eye care provider will take a few simple measurements for contact lenses. Contact lens measurements are quick and painless. If you so desire, your eye care provider can get lenses for you, or you can order them elsewhere. Some offices are very thorough and will ensure you are comfortable inserting and removing your lenses. And most importantly, taking care of them.