Bloodshot eyes may be related to visual fatigue, conjunctivitis, allergies and so on. 1. Visual fatigue: it may be long time reading or watching computer all the time, which causes visual fatigue, resulting in eye congestion and red bloodshot. 2. Conjunctivitis: when the conjunctiva is infected by pathogenic bacteria, due to the action of inflammatory factors on the blood vessels of the conjunctiva, the tension of the smooth muscle of the blood vessel wall is reduced. Thus, the diameter of blood vessels increases and blood flow increases, leading to redness in the eyes. 3. Allergy: common in allergic conjunctivitis and so on. When the conjunctival tissues of the eyes come into contact with allergens in the external environment, it will reactively cause vasodilatation and produce redness. There are many reasons for bloodshot eyes, so it is recommended to consult a doctor in a timely manner, follow the doctor’s instructions and then further relevant examinations, with the help of the doctor to clarify the cause, and targeted treatment or therapy.