Ocular redness, medically known as conjunctival injection or hyperemia, is a common and visible side effect of various substances introduced into the body. This phenomenon occurs when the delicate blood vessels covering the white part of the eye, called the conjunctiva, become engorged with blood. Many drugs, whether used recreationally or therapeutically, can trigger this physical change.
The Physiology of Drug-Induced Ocular Redness
The visible redness in the eye is a direct result of vasodilation, which is the widening of the tiny blood vessels and capillaries within the conjunctiva. When these vessels expand, the increased volume and flow of blood make them far more noticeable against the white background of the sclera.
Various chemical messengers released by the body can trigger this vascular response. For example, some drugs cause the release of histamine, a potent vasodilator involved in allergic and inflammatory reactions. Other substances may mimic or interfere with naturally occurring molecules like prostaglandins, which also mediate blood vessel dilation. Ultimately, any drug that causes a systemic drop in blood pressure or locally affects the vascular tone of the eye will likely induce this characteristic redness.
Recreational and Illicit Substances
Tetrahydrocannabinol (THC), the primary active compound in cannabis, is perhaps the most recognized cause of drug-induced ocular redness. THC binds to cannabinoid receptors throughout the body, including those in the eye, which leads to vasodilation. This systemic effect causes a drop in blood pressure, which in turn results in the dilation of the ocular capillaries, increasing blood flow and causing the bloodshot appearance. This mechanism is also responsible for the temporary reduction of intraocular pressure.
Alcohol is another common substance that causes bloodshot eyes through two distinct processes. First, alcohol acts as a systemic vasodilator, widening blood vessels across the body, including those in the eye, to create the red appearance. Second, alcohol is a diuretic, which promotes fluid loss and can lead to systemic dehydration. This dehydration affects the tear film, causing dry eye syndrome and subsequent irritation, which contributes to the redness.
Some recreational stimulants, such as cocaine and methamphetamine, can also lead to ocular redness, though often through secondary effects. While these drugs generally activate the sympathetic nervous system, causing pupil dilation and sometimes elevated blood pressure, they can also cause extreme dry eye due to reduced blinking. The resulting irritation and dryness can lead to the visible redness. Additionally, the method of drug use can directly irritate the eye surface, leading to conjunctival inflammation.
Prescription and Over-the-Counter Medications
A number of prescription medications can cause ocular redness as a known side effect, often due to their intended therapeutic action. Prostaglandin analogs (PGAs), a class of eye drops used to treat glaucoma, are a prime example. These drugs lower eye pressure by increasing the outflow of fluid, but they frequently cause conjunctival hyperemia because prostaglandins are powerful local vasodilators. This redness is a common reason why patients discontinue use.
Systemic vasodilators prescribed for conditions like high blood pressure can also cause blood vessels in the eye to widen. Any drug that lowers blood pressure by relaxing arterial smooth muscle can potentially affect the rich vascular supply of the eye. Certain non-vasodilator medications, such as bisphosphonates used for osteoporosis, can cause ocular redness by triggering inflammatory conditions like uveitis or scleritis.
A common cause of redness is the overuse of certain over-the-counter decongestant eye drops that contain vasoconstrictors. These drops temporarily shrink the blood vessels, making the eyes appear white. However, prolonged use can lead to a condition called rebound hyperemia, where the blood vessels become dependent on the drug and over-dilate when the effect wears off, resulting in redness that is worse than the original irritation.
Duration, Management, and Warning Signs
The duration of drug-induced ocular redness varies significantly depending on the substance and the dosage. Redness from acute recreational use, such as cannabis or alcohol, is typically transient and resolves naturally within a few hours as the substance is metabolized. Redness caused by prescription medications will persist as long as the medication is being used daily.
For temporary relief, simple non-medicated options can be helpful, such as applying a cold compress to the eyelids or using preservative-free artificial tears. These tears help lubricate the eye surface and flush out any irritants that may be contributing to the redness. Staying well-hydrated can also help mitigate dryness caused by diuretic substances like alcohol.
While most drug-induced redness is a benign cosmetic issue, certain symptoms warrant immediate medical attention. If the redness is accompanied by severe eye pain, a sudden change in vision, extreme sensitivity to light, or a persistent headache, it may indicate a more serious underlying issue. Similarly, redness that does not improve after discontinuing an over-the-counter drop or is a new side effect of a prescription drug should be discussed with a healthcare provider.