Tolerance, genetics, your body's ability to adapt. Maybe you're a mutant like a X-Man or from the show Heroes and your special ability is that your eyes don't get red when smoking the cheebah!

If I just get high my eyes don't get red but if I get baked my eyes turn to red rivers of blood vessels.

You put a frowning icon by the thread title. Does that mean you want your eyes to get red so that everyone now knows???????