Many medical patients and recreational users alike get their eyes red and dry, when they use weed. It’s not a reaction to smoke, as edibles will have the same effect. Rather, it’s the direct action of THC that makes the blood vessels in the eyes dilate and at the same time makes your eyes (and mouth) dry. As your tolerance to THC grows, the dry-eyes effect gets less noticeable. However, regardless of your tolerance levels, some strains are especially likely to cause dry eyes. Stay away from those and choose something else instead.