Contacts are generally very safe and well tolerated. They can cause problems such as infections, allergy, toxicity, or poor performance in vision correction. Most of these are rare if a careful clinician fits and monitors them.
Contacts are very convenient and helpful for vision correction but should not be the only means of correction for each patient. Patients who do not have glasses tend to wear their contacts eventhough they are bothering their eyes. This often leads to serious infections that can cause permanent damage to eyes. When in doubt get help to prevent injury.