Video and written answer:
Registered Optometrist Paul Carroll BSc(Hons) MCOptom MBA answers this question.

Lenses are safe as long as they are prescribed by a qualified practitioner, and as long as the wearer follows the practitioner's instructions for looking after them, returns for regular aftercare visits and seeks advice if they experience any problems. The eye is a sensitive and delicate structure so hygiene is important to ensure you do not suffer from eye infections. If you have any health problems, such as allergies, or take any medication, you need to tell your practitioner who will then advise you about how this affects your lens wear. If you experience any discomfort, redness or watering of the eyes while wearing lenses, you should take them out and not try to resume wear until the symptoms subside. If the symptoms persist or get worse see your optician urgently.

For more information about contact lenses, see our video below: