How does "whitening" eyedrops work?

Question

I use these almost every day, some "maximum redness relief"-version from the US, and I'm just curious how they work, what makes your eye almost change colour???

Answer 1

Drops that "get the red out" contain chemicals that are called vaso constrictors.  The most common one that is used is called phenylephrine.  When these drops are put in the eye they cause the muscles in walls of the blood vessles in your eyes to constrict.  This constriction narrows the vessels and makes them less visible.  These can be good drops to use on an occasional basis to deal with acute ALLERGIC redness.  If your problem is INFECTION, though, you are doing exactly the opposite of what you need to do.  By constricting the blood flow into the vessels you are decreasing the amount of you own antibodies in the blood that need to be in the eye to fight the infection.  If your problem is dryness, you are doing nothing to deal with the cause and eventually you will fatigue the blood muscles that are constricting and cause a phenomenon called rebound hyperemia which causes the vessels to be MORE visible.