Each spring, millions of people reach for over-the-counter (OTC) antihistamines to get relief from sneezing, congestion, and itchy eyes. Drugs like Claritin (loratadine), Zyrtec (cetirizine), and Allegra (fexofenadine) are widely used and considered safe by most standards. But while these medications are generally effective at suppressing allergy symptoms, they are not without potential side effects—especially when taken regularly or long-term.
This article explores the most common and lesser-known side effects of OTC antihistamines and what you should consider before relying on them as your primary allergy solution.
What Are Antihistamines?
Antihistamines work by blocking histamine, a chemical released by the immune system in response to allergens such as pollen, dust, and pet dander. Histamine causes inflammation and fluid buildup in the sinuses, eyes, and airways. By blocking this response, antihistamines reduce symptoms like sneezing, runny nose, and watery eyes.
There are two generations of antihistamines:
- First-generation antihistamines (like Benadryl) easily cross the blood-brain barrier and cause significant drowsiness.
- Second-generation antihistamines, such as Claritin and Zyrtec, are marketed as “non-drowsy” because they are less likely to enter the brain.
While second-generation drugs have fewer sedative effects, they are not completely free of side effects.
Common Side Effects of Claritin and Zyrtec
1. Drowsiness and Fatigue
Although less sedating than older antihistamines, both Claritin and Zyrtec can still cause drowsiness, especially in sensitive individuals or when taken in combination with other medications. Zyrtec, in particular, has a higher likelihood of causing sleepiness compared to Claritin.
A 2001 study published in Annals of Allergy, Asthma & Immunology found that nearly 14% of Zyrtec users experienced drowsiness, compared to 8% for Claritin users (Simons et al., 2001).
2. Brain Fog and Cognitive Effects
Many users report difficulty concentrating, slower thinking, and mental fatigue when taking antihistamines daily. These cognitive side effects may be subtle but can impact work performance, learning, and mood over time.
3. Dry Mouth, Nose, and Eyes
Antihistamines reduce histamine’s effects on mucous membranes, but this can also lead to over-drying. This dryness can affect:
- Saliva production (leading to bad breath or dental issues)
- Tear film in the eyes (causing irritation)
- Nasal passages (potentially worsening sinus discomfort)
4. Dizziness and Headaches
Some individuals experience lightheadedness, especially when first taking the medication or combining it with other drugs. Headaches are another commonly reported symptom.
Less Common But Concerning Effects
5. Digestive Issues
Both Claritin and Zyrtec may cause nausea, constipation, or abdominal discomfort, especially when taken on an empty stomach. These effects may be more noticeable in older adults or those with sensitive digestive systems.
6. Heart Palpitations and Increased Heart Rate
Though rare, some users report a racing heartbeat, especially with Claritin. This may be due to the drug’s slight stimulant effects or interactions with caffeine or other medications.
7. Mood Changes and Anxiety
While less well-studied, some users experience mood swings, irritability, or increased anxiety when using antihistamines long-term. This could be tied to changes in brain chemistry due to histamine blockade in the central nervous system.
Long-Term Use and Tolerance
Taking antihistamines daily over extended periods may lead to tolerance, meaning the medication becomes less effective over time. In response, users may increase their dose or switch brands, which can increase the risk of side effects.
There is also concern about rebound symptoms—worsening of allergy symptoms when stopping the medication suddenly after long-term use.
Antihistamines and the Elderly
Older adults may be especially vulnerable to the cognitive and sedative effects of antihistamines. The Beers Criteria, which guides safe medication use in older populations, advises caution when prescribing antihistamines to those over 65, due to increased risk of confusion, falls, and urinary retention (American Geriatrics Society, 2019).
Are There Alternatives?
For those looking to manage allergies without the side effects of OTC drugs, some natural alternatives include:
- Quercetin – a plant flavonoid that stabilizes mast cells and reduces histamine release
- Stinging nettle – shown in some studies to reduce allergy symptoms naturally
- NAC (N-acetylcysteine) – supports mucus clearance and reduces inflammation
- Saline nasal rinses – physically remove allergens and reduce congestion
- HEPA filters and environmental control – minimize exposure to triggers indoors
Always speak with a healthcare provider before stopping or replacing medication, especially during peak allergy season.
Conclusion
While OTC antihistamines like Claritin and Zyrtec are convenient and effective for many, they are not without side effects—ranging from drowsiness and dry mouth to cognitive impairment and mood changes. Awareness of these risks is especially important for long-term users or those seeking more holistic, sustainable approaches to allergy management. With the right tools, it’s possible to support your body’s natural defenses without relying solely on synthetic solutions.
References:
- Simons, F. E. R., Simons, K. J., & Frith, E. M. (2001). Comparative tolerability of cetirizine and loratadine. Annals of Allergy, Asthma & Immunology, 86(5), 617–622.
- American Geriatrics Society. (2019). Updated Beers Criteria for potentially inappropriate medication use in older adults. Journal of the American Geriatrics Society, 67(4), 674–694.