Antihistamines are commonly used medications to relieve allergy symptoms like sneezing, itching, and a runny nose. The potential for these drugs to affect pressure inside the eye, known as intraocular pressure (IOP), is a valid concern. While most modern allergy medications are safe for eye health, certain older types carry a rare risk of increasing IOP. This risk is associated with a pharmacological property that can interfere with the eye’s natural fluid drainage system.
Understanding Intraocular Pressure and Glaucoma Risk
Intraocular pressure (IOP) refers to the fluid pressure within the eye, which is maintained by a constant balance of fluid production and drainage. The eye produces a clear fluid called aqueous humor in the ciliary body, which then flows into the front of the eye. This fluid provides nutrients to the eye’s structures and helps maintain the eye’s spherical shape.
The aqueous humor drains out of the eye primarily through a spongy tissue called the trabecular meshwork, located at the angle where the iris and cornea meet. If the production of this fluid exceeds the rate of its drainage, the IOP rises. Glaucoma is a group of eye conditions characterized by damage to the optic nerve, which is often linked to this pathologically high IOP.
The Anticholinergic Mechanism Behind IOP Changes
The mechanism by which some antihistamines elevate IOP is tied to their anticholinergic properties. These properties block the action of the neurotransmitter acetylcholine, which affects involuntary muscle movements, including those in the eye. In the eye, this action causes the sphincter muscle of the iris to relax, leading to pupil dilation (mydriasis).
For individuals with a naturally narrow angle between the iris and cornea, this dilation is problematic. The widening of the pupil causes the peripheral iris tissue to bunch up, physically obstructing the trabecular meshwork. This blockage prevents the aqueous humor from draining, leading to a sudden and dangerous spike in IOP, termed an acute angle-closure crisis. This rapid pressure rise can cause severe damage to the optic nerve if not treated immediately.
Navigating Risk: First-Generation vs. Second-Generation Antihistamines
The risk of elevated IOP differs strongly between the two main classes of antihistamines. First-generation antihistamines are older compounds that readily cross the blood-brain barrier and have significant anticholinergic activity. This high anticholinergic load makes them the primary concern for triggering an acute angle-closure crisis in susceptible individuals. Examples include diphenhydramine (Benadryl) and chlorpheniramine.
Second-generation antihistamines are newer agents with a much lower or negligible anticholinergic effect. These drugs are more selective and generally do not cross the blood-brain barrier effectively, minimizing systemic side effects like drowsiness and ocular effects. Consequently, medications such as cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) carry a very low risk of affecting IOP, even for people with pre-existing eye conditions.
When to Consult an Eye Care Professional
The risk from antihistamines is concentrated in individuals with pre-existing anatomical factors, specifically a narrow angle or diagnosed angle-closure glaucoma. Those who have been told they have narrow angles should be particularly cautious with first-generation antihistamines and other anticholinergic drugs.
Consult an eye care professional immediately if you experience symptoms of an acute angle-closure crisis after taking any medication. These symptoms include the sudden onset of severe eye pain, blurred or cloudy vision, seeing rainbow-like halos around lights, or accompanying headache or nausea. Always inform your pharmacist and physician about your eye health history, including any diagnosis of glaucoma, before starting any new medication.