Clemastine is a first-generation antihistamine that belongs to the ethanolamine class of compounds, which also includes diphenhydramine (Benadryl). Like other first-generation antihistamines, it is effective at providing relief for a variety of allergic conditions but is known for causing significant drowsiness. It works by blocking the action of histamine, a chemical released by the body during an allergic reaction. While it was once widely available over-the-counter, its status has changed, and it is now primarily a prescription medication in the U.S..
Common Trade Names for Clemastine
Several trade names have been associated with clemastine over the years. Some are still in use, while others have been discontinued or are available in different forms or countries.
- Tavist® and Tavist® Allergy: This is one of the most widely recognized trade names for clemastine. The over-the-counter (OTC) version of Tavist, particularly Tavist-1, has been discontinued in the U.S., where clemastine is now available only by prescription in higher-strength doses. In the past, Tavist was marketed for allergy relief and was also included in combination products for allergy and sinus symptoms.
- Dayhist® Allergy: Another brand name for clemastine, which was also previously available over-the-counter. Like Tavist, the availability of Dayhist in the U.S. as an OTC product has ceased, and clemastine in any form is now a prescription-only medication.
- Tavegyl: While less common in the U.S., Tavegyl is a brand name for clemastine that is recognized internationally. It is used for similar purposes, including treating allergic rhinitis and skin conditions.
- Generic Clemastine: In addition to brand names, clemastine is also available in generic formulations, typically as tablets or syrup. These generic versions contain the same active ingredient and are available by prescription.
How Clemastine Works in the Body
Clemastine is classified as a competitive histamine H1 receptor antagonist. Its mechanism of action involves the following steps:
- Binding to H1 Receptors: During an allergic reaction, the body's immune system releases histamine, which binds to H1 receptors on cells, causing symptoms like itching, sneezing, and a runny nose. Clemastine competes with histamine for these same receptor sites.
- Blocking Histamine: By binding to the H1 receptors, clemastine blocks the action of histamine, preventing it from triggering allergic symptoms. It does not prevent the release of histamine but rather stops it from having its effect once released.
- Achieving Symptom Relief: The blocking of H1 receptors leads to a reduction in allergy symptoms, including sneezing, runny nose, itchy and watery eyes, and itching and swelling associated with hives.
- Sedative and Anticholinergic Effects: As a first-generation antihistamine, clemastine crosses the blood-brain barrier, affecting the central nervous system (CNS). This causes its characteristic sedative side effect. It also possesses anticholinergic properties, which contribute to other side effects like dry mouth and blurred vision.
Potential Side Effects and Precautions
Like all medications, clemastine is associated with a range of side effects. Its sedative effects are particularly notable, and it should be used with caution, especially in certain populations.
Common Side Effects
- Drowsiness and sedation
- Dizziness and fatigue
- Dry mouth, nose, and throat
- Decreased coordination
- Nausea and upset stomach
- Headache
Serious Side Effects (Consult a doctor immediately)
- Difficulty urinating or changes in urination
- Changes in vision, including blurred vision
- Fast, pounding, or irregular heartbeat (tachycardia)
- Hives, rash, or severe itching
- Difficulty breathing or swallowing
- Paradoxical CNS stimulation, including excitement and hallucinations (especially in children)
Precautions
- Alcohol and CNS Depressants: The sedative effects of clemastine are enhanced by alcohol and other CNS depressants, such as tranquilizers and sleeping pills. This combination should be avoided.
- Activities Requiring Alertness: Due to the risk of drowsiness and impaired coordination, patients should avoid driving or operating heavy machinery until they understand how the medication affects them.
- Elderly Patients: Older adults are more susceptible to the side effects of antihistamines, particularly dizziness, sedation, and hypotension. Clemastine is generally not recommended for this population.
- Specific Medical Conditions: Use with caution in patients with narrow-angle glaucoma, asthma, cardiovascular disease, hypertension, hyperthyroidism, or difficulty urinating due to an enlarged prostate.
Comparison of Clemastine with Newer Antihistamines
First-generation antihistamines like clemastine differ significantly from the newer, second-generation and third-generation alternatives. The following table highlights some key differences:
| Feature | Clemastine (First-Generation) | Loratadine (Second-Generation Example) |
|---|---|---|
| Mechanism | Blocks H1 receptors both peripherally and centrally. | Primarily blocks H1 receptors peripherally. |
| Sedation | Causes significant drowsiness and fatigue due to crossing the blood-brain barrier. | Non-drowsy or low-sedation, as it does not readily cross the blood-brain barrier. |
| Anticholinergic Effects | Strong anticholinergic properties (e.g., dry mouth, blurred vision). | Minimal anticholinergic effects. |
| Drug Interactions | Significant interactions with alcohol, CNS depressants, and MAO inhibitors. | Fewer drug interactions compared to first-generation antihistamines. |
| Onset of Action | Typically takes 2-4 hours for peak effect. | Also has a relatively fast onset, but with a different side effect profile. |
| Duration of Action | Long-acting, with effects lasting up to 12 hours or more. | Long-acting (e.g., 24-hour relief). |
| Availability (U.S.) | Prescription only. | Available over-the-counter. |
Conclusion
Clemastine, known by the trade names Tavist and Dayhist, is an older, first-generation antihistamine that effectively treats a variety of allergy symptoms. However, its use is associated with considerable drowsiness and other anticholinergic side effects due to its ability to affect the central nervous system. In the U.S., it is no longer available over-the-counter and is now a prescription-only medication. For many people seeking allergy relief, newer, non-drowsy second-generation antihistamines are often preferred due to their more favorable side effect profile. Patients should always consult a healthcare provider to determine the most appropriate and safest treatment option for their specific allergy needs.
For more detailed prescribing information and drug details, you can visit the DailyMed page for Clemastine Fumarate Syrup.
