Bloodshot eyes, characterized by redness on the white part of the eye, occur when tiny blood vessels on the eye’s surface become visibly dilated. This common phenomenon can result from various factors, including environmental irritants, underlying health conditions, and the use of certain substances. Understanding the different causes behind this redness can provide insight into its temporary or persistent nature.
The Underlying Cause of Red Eyes
The eye’s visible white surface, known as the sclera, is covered by a clear membrane called the conjunctiva. Within this delicate membrane lie numerous small blood vessels that are typically not prominent. When these vessels widen, they become more noticeable, leading to the characteristic red or “bloodshot” appearance. This dilation is often a response to irritation, inflammation, or changes in local blood flow, which can be triggered by a range of internal and external stimuli.
Recreational Substances and Eye Redness
Various recreational substances can induce eye redness through different physiological mechanisms. The effects on blood vessels and tear production often contribute to this noticeable side effect.
Cannabis is well-known for causing bloodshot eyes. Tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, causes vasodilation, meaning it widens blood vessels throughout the body, including those in the eyes. This increased blood flow makes the vessels more visible, leading to a red appearance. The effect is not due to smoke irritation but rather THC’s systemic impact.
Alcohol consumption can also result in bloodshot eyes. Alcohol acts as a diuretic, leading to dehydration throughout the body, including the eyes. This dehydration can cause dry, irritated eyes, which may appear red. Alcohol also causes blood vessels, including those in the eyes, to swell and fill with blood, contributing to the bloodshot appearance.
Stimulants such as cocaine and amphetamines can sometimes lead to eye redness, though their primary ocular effects often involve pupil dilation. While these substances initially cause vasoconstriction, a rebound effect can occur as they wear off, leading to vasodilation. Stimulants can also cause dry eyes, which contributes to irritation and a red appearance.
Opioids and other depressants are less commonly associated with eye redness, often causing constricted pupils instead. Individuals may experience dry eyes or mild irritation from these substances, leading to redness. This effect is generally less direct compared to the vasodilation seen with cannabis or the dehydration from alcohol.
Medications That Can Cause Bloodshot Eyes
Prescription and over-the-counter medications can also contribute to eye redness, often as an unintended side effect. These effects stem from changes in tear production or blood vessel regulation.
Antihistamines, commonly used for allergies, can cause dry eyes. Both oral antihistamines and some nasal sprays can reduce tear production. This drying effect can make the eyes feel gritty, scratchy, and appear red.
Oral decongestants, used to relieve nasal congestion, can also contribute to dry eyes. Like antihistamines, these medications can reduce overall body fluid, including the moisture needed for healthy tear film production. This can result in eye irritation and redness.
Certain blood pressure medications work by widening blood vessels throughout the body. This systemic effect can extend to the eyes, causing the small blood vessels in the conjunctiva to dilate and become more prominent, leading to a bloodshot appearance. Beta-blockers can also cause dry eyes by reducing tear production.
Some medicated eye drops can cause redness. For example, certain glaucoma medications may cause irritation or have preservatives that lead to eye surface inflammation.
Acne medications are known for causing dry eyes. This medication reduces the activity of sebaceous glands. A compromised tear film leads to increased tear evaporation, causing dryness, irritation, and redness.
When to Be Concerned About Red Eyes
While many instances of bloodshot eyes are temporary and harmless, certain accompanying symptoms warrant medical attention. If eye redness persists for more than a day or two, or if it is accompanied by pain, sensitivity to light, or changes in vision, a healthcare provider should be consulted.
Other concerning symptoms include discharge, a feeling that something is in the eye, or swelling of the eyelids. These signs might indicate an underlying condition such as an infection (e.g., conjunctivitis), inflammation (e.g., uveitis), or even an injury. Seeking professional evaluation can help rule out more serious eye conditions.