The appearance of bloodshot eyes is a widely recognized physiological reaction following cannabis use. This phenomenon, which causes the white sclera to take on a reddish hue, is a predictable biological side effect. The color change results directly from chemical interactions that affect the body’s circulatory system. Understanding this requires examining the systemic mechanisms governing blood flow and pressure.
The Chemical Cause of Blood Vessel Dilation
The core reason for the redness lies with Delta-9-tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis. When THC enters the bloodstream, it interacts with the body’s endocannabinoid system, specifically activating CB1 receptors in the vascular system. This activation triggers vasodilation, which is the widening of blood vessels. This action immediately causes a temporary drop in overall systemic blood pressure, known as hypotension.
The body responds to this pressure decrease by relaxing and expanding blood vessels throughout the circulatory network. This systemic widening reduces resistance to blood flow. Since the effect is systemic, it is present regardless of the method of consumption, whether inhaled or ingested. The degree of this reaction is directly related to the amount of THC consumed, meaning higher concentrations produce a more pronounced hypotensive effect.
How Vasodilation Creates Visible Redness
Systemic vasodilation translates into visible redness when it reaches the fine blood vessels of the eye. The sclera, the white part of the eye, is covered by the conjunctiva, a membrane containing a network of tiny capillaries. As blood pressure drops due to THC’s effects, these conjunctival blood vessels relax and expand significantly.
When these capillaries widen, they allow a greater volume of blood to flow through them. This increased blood volume and enlarged diameter make the vessels conspicuous against the white background of the eye, resulting in the characteristic bloodshot look.
A related consequence of this vasodilation is the reduction of intraocular pressure (IOP). The widening of ocular capillaries facilitates better drainage of the aqueous humor, the fluid that fills the front part of the eye. This reduction in IOP is a direct result of the same physiological mechanism that causes the visible redness. The redness is simply the visual manifestation of the body’s temporary adjustment to the presence of THC.
Practical Ways to Reduce Eye Redness
Since the redness is caused by a chemical process, it naturally subsides as the body metabolizes THC and blood pressure returns to normal, typically within a few hours. For a quicker solution, over-the-counter redness-reducing eye drops are widely available. These products contain a vasoconstrictor ingredient.
These specialized drops temporarily reverse vasodilation by causing the capillaries in the eye to constrict. This quickly reduces visible blood flow, making the eyes appear white again. Note that these drops only address the symptom and do not change the underlying chemical effect of the THC.
Other Remedies
Other simple remedies can help mitigate the appearance of redness. Applying a cool compress to the closed eyelids can cause the surface blood vessels to constrict slightly. Staying well-hydrated may also help, as dehydration can exacerbate eye dryness and irritation. Choosing cannabis products with a lower concentration of THC can also reduce the intensity and duration of the bloodshot eye effect.