Can Allergy Meds Cause Constipation? Understanding the Link

October 9, 2025 •

4 min read

First-generation antihistamines are a class of medications known to have a higher risk for anticholinergic side effects like constipation. This happens because these medications interfere with nerve signals in your gut that help regulate digestion. While newer, second-generation allergy meds are much less likely to cause this issue, it's a known concern for many users.

Quick Summary

Allergy medications, particularly older antihistamines, can cause constipation by blocking a nerve signal that controls gut movement. Newer versions pose a lower risk. Lifestyle changes like increasing fluid intake and fiber can help manage symptoms, but it is important to consult a healthcare provider for persistent issues.

Key Points

Antihistamines can cause constipation: First-generation antihistamines, like Benadryl, are known to have constipation as a potential side effect due to their anticholinergic properties.
Anticholinergic effects slow digestion: The mechanism involves blocking acetylcholine, a neurotransmitter that stimulates muscle contractions in the gut (peristalsis).
Newer meds have a lower risk: Second-generation antihistamines, such as Claritin and Zyrtec, have fewer anticholinergic effects and are less likely to cause constipation.
Lifestyle changes can help: Increasing water intake, eating more fiber, and exercising regularly are effective strategies to manage or prevent medication-induced constipation.
Consult a professional before changing meds: You should never stop or change a medication without first speaking to your doctor or pharmacist, as they can recommend safer alternatives or management techniques.
Recognize serious symptoms: If constipation is severe, persistent, or accompanied by other worrying symptoms, seek immediate medical advice.

The Surprising Connection: How Allergy Meds Affect Your Digestion

When you think of allergy medication side effects, drowsiness or dry mouth are often the first to come to mind. However, many people are unaware that these same drugs can also lead to constipation. The mechanism behind this side effect is related to how different generations of antihistamines interact with the nervous system, particularly the gut's involuntary muscle movements.

The Role of Anticholinergic Effects

The key to understanding how allergy meds cause constipation lies in their anticholinergic properties. Anticholinergic drugs block the action of a neurotransmitter called acetylcholine. In the digestive system, acetylcholine plays a critical role in promoting peristalsis, the muscle contractions that move food through the intestines. By blocking this chemical messenger, antihistamines can significantly slow down gut motility, causing food and waste to move more sluggishly. This slower transit time allows more water to be absorbed from the stool, leading to harder, drier bowel movements and, ultimately, constipation.

First-Generation vs. Second-Generation Antihistamines

The likelihood of experiencing this side effect largely depends on the type of antihistamine you take. Antihistamines are generally divided into two generations, with significant differences in their anticholinergic activity.

First-Generation (Sedating) Antihistamines

First-generation antihistamines were developed first and are known for causing more sedation. This is because they can cross the blood-brain barrier and affect the central nervous system more directly.

Higher Risk: Medications like diphenhydramine (Benadryl) have strong anticholinergic properties, making them more likely to cause constipation, dry mouth, and urinary retention.
Examples: Common over-the-counter options include Benadryl (diphenhydramine) and Chlor-Trimeton (chlorpheniramine).
Other Side Effects: In addition to constipation, they can cause significant drowsiness, dizziness, and blurred vision.

Second-Generation (Non-Sedating) Antihistamines

Second-generation antihistamines were specifically designed to minimize sedation and other unwanted side effects. They are less likely to cross the blood-brain barrier and have fewer anticholinergic effects.

Lower Risk: Medications like loratadine (Claritin), cetirizine (Zyrtec), and fexofenadine (Allegra) have a much lower risk of causing constipation.
Examples: Allegra (fexofenadine), Claritin (loratadine), and Zyrtec (cetirizine) are popular examples.
Preferred Option: For individuals sensitive to the anticholinergic effects of older drugs, these are often the preferred choice for managing seasonal allergies.

Comparison of Antihistamine Generations and Constipation Risk


Feature	First-Generation Antihistamines	Second-Generation Antihistamines
Constipation Risk	Higher risk due to strong anticholinergic effects.	Lower risk due to fewer anticholinergic effects.
Mechanism	Block acetylcholine, slowing intestinal muscle contractions.	Minimal blocking of acetylcholine.
Common Examples	Benadryl (diphenhydramine), Chlor-Trimeton (chlorpheniramine).	Claritin (loratadine), Zyrtec (cetirizine), Allegra (fexofenadine).
Sedation	Common, as they cross the blood-brain barrier.	Rare, as they do not readily cross the blood-brain barrier.
Other Side Effects	Dry mouth, blurred vision, urinary retention.	Typically mild side effects, if any.

Management and Prevention Strategies

If you find yourself experiencing constipation from your allergy medication, there are several steps you can take to manage or prevent it. It's crucial to first consult with your healthcare provider and never stop taking a prescribed medication without their approval.

Hydration is Key: Increasing your fluid intake is one of the most effective strategies. Drinking plenty of water helps soften stool, making it easier to pass.
Increase Dietary Fiber: Adding more fiber-rich foods to your diet can aid in digestion. Good sources include fruits (especially pears and berries), vegetables (broccoli, leafy greens), and whole grains. Fiber supplements like psyllium husk can also be beneficial.
Regular Exercise: Physical activity can stimulate the intestinal muscles, promoting regular bowel movements. Even a daily walk can help get things moving.
Switch Medications: If you are using a first-generation antihistamine, your doctor may recommend switching to a second-generation option, which has a lower risk of causing constipation.
Consider OTC Treatments: For temporary relief, over-the-counter options such as stool softeners (docusate sodium) or osmotic laxatives (polyethylene glycol) can be effective. Always discuss these with a healthcare professional first.

When to Seek Medical Advice

While constipation from allergy meds is often manageable with lifestyle changes, there are situations that warrant a call to your doctor. Seek medical help if you experience:

Severe or persistent constipation.
Abdominal pain or cramping.
Nausea or vomiting.
Blood in your stool.
No bowel movement for more than a week.

Conclusion

Yes, certain allergy medications, particularly older antihistamines, can cause constipation due to their anticholinergic effects, which slow down intestinal motility. However, newer, non-sedating antihistamines are much less likely to cause this issue. Managing and preventing medication-induced constipation often involves simple lifestyle adjustments like increasing fluid intake and dietary fiber, as well as regular exercise. If symptoms persist or become severe, it is important to consult a healthcare provider to discuss alternative medication options or other treatments. By understanding the potential for this side effect, you can take proactive steps to maintain your digestive health while effectively managing your allergy symptoms.

Frequently Asked Questions

First-generation antihistamines are most likely to cause constipation. Common examples include diphenhydramine (Benadryl) and chlorpheniramine (Chlor-Trimeton). Newer, second-generation antihistamines are much less likely to cause this side effect.

These medications cause constipation due to their anticholinergic properties. They block the neurotransmitter acetylcholine, which plays a crucial role in regulating gut muscle contractions. This leads to slower intestinal movement and harder stools.

Yes, second-generation antihistamines like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) are generally a better choice. They have fewer anticholinergic effects and therefore a much lower risk of causing constipation.

Effective home remedies include increasing your daily water and fluid intake, eating more high-fiber foods like fruits, vegetables, and whole grains, and engaging in regular physical exercise. Fiber supplements may also help.

Yes, over-the-counter stool softeners (like docusate sodium) or osmotic laxatives (like polyethylene glycol) can help. However, you should always consult a healthcare provider or pharmacist before starting any new medication to ensure it is appropriate for you.

If you started experiencing constipation shortly after beginning a new allergy medication, or if the constipation improves when you stop taking it, it may be the cause. It is important to discuss this with a doctor to rule out other possible reasons for your symptoms.

You should never stop taking a prescribed medication without first consulting your doctor. A healthcare provider can help you manage the side effects, suggest an alternative medication, or recommend an over-the-counter remedy.