My toddler has had a one red eye that only covers half of his eye for several weeks now. It clears up a little sometimes but then goes back to being red on the one half. No discharge or light sensitivity. Antibiotic drops and medicine has not helped. His doctor is now saying to use "Zaditor" and to see eye dr. if this does not clear it up by in a few days. Is this just from allergies or could it be more serious?