The phenomenon commonly known as “red eyes” is medically termed conjunctival hyperemia, describing the dilation and engorgement of the blood vessels covering the white part of the eye. This visible redness is not a condition itself but a symptom indicating a change in the eye’s vascular system, often triggered by a localized irritant or systemic substances. Many drugs, both prescription and recreational, can induce this effect by interacting with the body’s circulatory and nervous systems. Understanding the underlying biological mechanisms reveals why certain substances consistently lead to this sign.
The Biological Mechanism of Ocular Redness
Ocular redness occurs when the delicate blood vessels in the conjunctiva—the thin, transparent membrane lining the eyelid and covering the sclera—widen significantly. This process, known as vasodilation, causes the vessels to become engorged with blood and far more visible against the white background of the eye. The conjunctiva contains an intricate network of microvasculature responsive to chemical signals and changes in blood flow.
When a drug or irritant is introduced, the body often releases signaling molecules like nitric oxide or histamine. These compounds relax the smooth muscle cells within the vessel walls, increasing the diameter of the capillaries. The resulting surge in blood volume and flow produces the distinctive, bloodshot appearance. This dilation often serves the physiological purpose of increasing blood supply to deliver healing agents or flush out irritants.
Substances That Cause Systemic Vasodilation
Many widely used substances cause redness through a systemic effect, meaning the drug enters the bloodstream and affects the entire body’s circulatory system, including the eyes. The psychoactive compound delta-9-tetrahydrocannabinol (THC) found in cannabis is a well-known example that achieves this effect through direct pharmacological action. THC binds to cannabinoid receptors located throughout the body, including those within the ocular tissue.
This binding directly triggers vasodilation in the conjunctival blood vessels, which leads to immediate, visible redness. THC is also a systemic vasodilator, lowering overall blood pressure. This mechanism contributes to its effect of reducing intraocular pressure (IOP). This decrease in IOP, a factor that benefits patients with glaucoma, is a direct result of the same vascular changes that cause the eyes to become bloodshot. The redness is often dose-dependent and occurs regardless of the method of consumption, confirming it is not merely irritation from smoke.
Alcohol is another common systemic vasodilator that affects the appearance of the eyes. Upon consumption, alcohol is metabolized, and its byproducts cause the smooth muscles in blood vessel walls across the body to relax and widen. This increased blood flow becomes most apparent in the small vessels of the eyes, leading to a bloodshot appearance.
In some individuals, this effect is exacerbated by a genetic variation that hinders the metabolism of acetaldehyde, a toxic compound produced when the body breaks down alcohol. This accumulation causes a severe flushing reaction, including pronounced ocular redness, often called the “alcohol flush reaction.” Dehydration, a secondary effect of alcohol consumption, also contributes to redness by reducing tear film production, causing surface irritation and dry eye.
Medications Causing Direct or Rebound Irritation
While systemic drugs act from within, many topical and some oral medications cause redness through localized irritation or a specific rebound effect. Over-the-counter (OTC) decongestant eye drops, which contain vasoconstrictors like tetrahydrozoline or naphazoline, are a prime example. These drops temporarily constrict the conjunctival blood vessels, reducing redness.
The issue arises with overuse, which leads to rebound hyperemia. As the drug wears off, the blood vessels react by dilating excessively, often becoming wider and redder than they were before the drops were applied. This paradoxical effect forces users to apply the drops more frequently, creating a cycle of dependency and chronic redness.
Certain classes of systemic prescription medications can also induce redness through indirect mechanisms, such as causing severe dry eye. Oral antihistamines, for instance, reduce tear production due to their anticholinergic properties, leading to ocular surface dryness and irritation. Additionally, some beta-blockers may alter the eye’s vascular tone or tear composition, contributing to irritation. Even preservatives in eye drops, such as benzalkonium chloride, can disrupt the natural tear film and damage surface cells, causing inflammation and visible redness.