From the Guidelines
Clemastine is a first-generation antihistamine that should be used with caution due to its potential for sedation and anticholinergic side effects, and its use is generally recommended for nighttime allergy relief. Clemastine works by blocking histamine H1 receptors, preventing histamine from binding to these receptors and causing allergic reactions 1. It is available as an over-the-counter medication in many countries, typically in tablet or syrup form, with common dosages ranging from 1.34 to 2.68 mg taken once or twice daily, as indicated in the package insert 1. The medication begins working within 1-3 hours and its effects can last for 10-12 hours. Some key points to consider when prescribing or using clemastine include:
- Its sedative effects, which can be beneficial for nighttime allergy relief but problematic for daytime use when alertness is required 1
- Potential side effects such as dry mouth, blurred vision, constipation, and urinary retention due to its anticholinergic properties 1
- The need for caution in patients with glaucoma, enlarged prostate, or urinary retention issues 1
- Comparison with second-generation antihistamines, which may have fewer sedative effects but similar efficacy in relieving allergy symptoms 1. Some of the key characteristics of clemastine include:
- Half-life: 21.3 ± 11.6 hours 1
- Sedation/somnolence or CNS impairment: 56.67% 1
- Age limit: 6 years old 1
- Dosage forms: 1.34,2.68 mg tablets or syrup 1
- Adult dose: 1.34 mg to 2.68 mg, taken twice or three times a day 1.
From the FDA Drug Label
Clemastine fumarate is an antihistamine with anticholinergic (drying) and sedative side effects. Antihistamines appear to compete with histamine for cell receptor sites on effector cells. The action of clemastine is as an antihistamine, competing with histamine for cell receptor sites on effector cells, with additional anticholinergic and sedative effects.
- The antihistaminic activity of clemastine reaches a peak at 5-7 hours and persists for 10-12 hours, and in some cases, for as long as 24 hours 2
From the Research
Clemastine Mechanism of Action
- Clemastine is a first-generation antihistamine that works by inhibiting the action of histamine, a chemical released by the body's immune system during an allergic reaction 3.
- It has been shown to suppress histamine sensitivity of both skin and nasal mucosa, although its antiallergic activity is lower compared to newer generation antihistamines such as fexofenadine and cetirizine 3.
- Clemastine has also been found to cause immune suppression through inhibition of extracellular signal-regulated kinase-dependent proinflammatory cytokines, which could be exploited to treat inflammatory diseases 4.
Therapeutic Potential
- Clemastine has been recognized as a potential therapeutic agent for various neurological disorders, including neurodegenerative diseases, neurodevelopmental deficits, brain injury, and psychiatric disorders 5.
- It has been shown to transport across the blood-brain barrier and act on specific neurons and neuroglia to exert its protective effect 5.
- Clemastine is also used to treat allergic conditions such as allergic rhinitis and urticaria, although newer generation antihistamines are generally preferred due to their safer adverse effect profile 6.
Safety Profile
- Clemastine has been found to be a high potency inhibitor of the HERG K+ channel, which could potentially lead to delayed ventricular repolarization and QT interval prolongation 7.
- However, no adverse effects on the human QT interval have been reported, highlighting the need to interpret HERG IC50 data in the context of other safety assays 7.
- The safety profile of antihistamines like clemastine may need to be revisited, particularly in light of their potential immunosuppressive effects 4.