Seasonal allergies (hay fever) are usually associated with respiratory symptoms like sneezing and watery eyes. However, many people experience digestive issues, including constipation, during peak allergy seasons. This connection arises from biological responses, medication side effects, and changes in daily habits. Understanding the underlying mechanisms helps in finding relief.
Physiological Links Between Allergies and Gut Motility
The allergic response involves releasing chemicals like histamine from mast cells, which are immune cells found throughout the body, including the digestive tract. When an allergen is encountered, this systemic inflammatory cascade affects areas beyond the nasal passages. This widespread signaling can disrupt the enteric nervous system, which controls gut function.
The presence of histamine and inflammatory cytokines in the gut alters the normal rhythm of peristalsis, the muscular contractions that move waste through the intestines. This disruption slows the transit time of stool, allowing more water to be reabsorbed. The resulting drier, harder stool is more difficult to pass, manifesting as constipation.
The Impact of Allergy Medications on Digestion
The most frequent cause of allergy-related constipation is the use of certain over-the-counter medications. First-generation antihistamines, such as diphenhydramine, possess strong anticholinergic properties. This means they block acetylcholine, a neurotransmitter that powers the parasympathetic nervous system, often called the “rest and digest” system.
Blocking acetylcholine reduces the involuntary smooth muscle contractions in the intestinal wall necessary for peristalsis. This slows the gut down, leading to reduced movement and constipation. Decongestants, like pseudoephedrine, also contribute by stimulating the sympathetic nervous system. This activation redirects the body’s resources away from digestion, further inhibiting normal gut function.
Secondary Factors Contributing to Constipation
Beyond physiological and pharmacological effects, several behavioral and environmental factors during allergy season can exacerbate constipation. Dehydration is common, as the body uses fluid resources to produce the excess mucus characteristic of hay fever. Insufficient fluid intake causes the body to absorb more water from the colon, resulting in harder stools.
Severe allergy symptoms often lead to reduced physical activity, as people stay indoors to avoid high pollen counts. Lack of movement slows intestinal transit, a primary factor in constipation. Furthermore, the stress and poor sleep accompanying chronic allergy symptoms disrupt the gut’s natural rhythm, as the gut-brain axis is sensitive to stress hormones.
Managing Constipation While Treating Allergies
The most effective strategy involves selecting allergy treatments that minimize digestive side effects. Switching from older first-generation antihistamines to newer, second-generation options (like fexofenadine or cetirizine) is recommended. These newer medications have minimal anticholinergic activity, which lowers the risk of medication-induced constipation. Using topical treatments, such as medicated nasal sprays, can also reduce the need for oral systemic medications.
Non-medication strategies help maintain regularity. Increasing daily fluid intake is important to counteract dehydration caused by allergy symptoms and drying medication side effects. Incorporating more dietary fiber and engaging in gentle physical activity encourages normal peristaltic motion, supporting digestive health during allergy season.