Why does smoking marijuana make your eyes red?

Marijuana decreases pressure in the eyes and increases blood flow. When this happens, the arteries in the eye-ball expand (due to decreased pressure). These areteries, especially when expanded, are visible in the white portion of the eye and make the eye appear "bloodshot" or red.

This very result (decrease of pressure in the eye) is the primary reason marijuana is used to treat glaucoma. Glaucoma is the condition where there is excessive pressure in the eye. In some patients, the pressure can become so severe that it causes damage to the retinal nerve, ultimately damaging the patient's vision. Marijuana decreases this pressure.