Why should we take antihistamines (anti-allergic medications) at night?

Taking Antihistamines at Night: Benefits and Risks

First-generation antihistamines should only be taken at night when a sedative effect is specifically desired, while second-generation antihistamines are generally preferred for daytime use due to their minimal sedative effects. 1

Types of Antihistamines and Their Sedative Properties

First-Generation Antihistamines

• Examples: diphenhydramine, hydroxyzine, clemastine, chlorphenamine
• Cross the blood-brain barrier easily
• Significant sedative and anticholinergic effects
• Sedation can persist into the next day (hangover effect)
• PET studies show 44.7% H₁ receptor occupancy in the brain the morning after nighttime diphenhydramine use 2

Second-Generation Antihistamines

• Examples: fexofenadine, loratadine, desloratadine, cetirizine
• Varying degrees of sedation:
  • Non-sedating at recommended doses: fexofenadine, loratadine, desloratadine
  • Potentially sedating at recommended doses: cetirizine, intranasal azelastine 1

When Night Administration Is Appropriate

Taking antihistamines at night may be appropriate in these specific situations:

1. When sedation is desired to help with sleep:

   • First-generation antihistamines may be prescribed at bedtime when a soporific (sleep-inducing) effect is desired 1
   • This can be helpful for patients with allergic conditions who also have difficulty sleeping

2. When anticholinergic effects are beneficial:

   • For patients with persistent rhinorrhea (runny nose) despite other treatments 1
   • However, topical anticholinergic agents are generally preferred over systemic ones when possible

Risks of Taking Sedating Antihistamines at Night

Despite common practice, there are significant concerns with nighttime use of sedating antihistamines:

1. Next-day impairment:

   • First-generation antihistamines taken at bedtime can cause significant daytime drowsiness, decreased alertness, and performance impairment the next day 1
   • This occurs because these medications and their metabolites have prolonged plasma half-lives 1

2. Persistent cognitive effects:

   • Performance impairment can exist the next morning without subjective awareness of drowsiness 1
   • This creates a dangerous situation where patients may engage in activities requiring alertness while impaired

3. No proven benefit for urticaria treatment:

   • Research shows that adding a sedating antihistamine at night to a non-sedating one during the day does not improve treatment efficacy in chronic urticaria, but does increase daytime somnolence 3

Special Populations at Risk

Certain populations are at higher risk for adverse effects from sedating antihistamines:

1. Older adults:

   • More sensitive to psychomotor impairment
   • Increased risk for falls, fractures, and subdural hematomas
   • More susceptible to anticholinergic effects 1

2. Patients with comorbid conditions:

   • Increased intraocular pressure
   • Benign prostatic hypertrophy
   • Preexisting cognitive impairment 1

3. Individuals who drive or operate machinery:

   • Drivers taking first-generation antihistamines are 1.5 times more likely to be responsible for fatal accidents 1

Best Practices for Antihistamine Use

1. For allergic rhinitis:

   • Second-generation antihistamines are generally preferred due to minimal sedation and anticholinergic effects 1
   • If nighttime symptoms are problematic, consider using a second-generation antihistamine consistently rather than switching to a first-generation one at night

2. For urticaria:

   • Guidelines recommend non-sedating second-generation antihistamines as first-line treatment 1
   • Increasing the dose of a second-generation antihistamine is preferred over adding a sedating one at night 3

3. For sleep issues in allergic conditions:

   • Consider that antihistamines have not been demonstrated to improve pruritus in randomized controlled trials 1
   • Improvement in nocturnal symptoms is likely due to sedative effect rather than anti-allergic properties 1

Common Pitfalls to Avoid

1. AM/PM dosing regimen: Combining a second-generation agent in the morning with a first-generation agent at night is not recommended as it does not avoid daytime drowsiness and performance impairment 1

2. Tolerance development: Children can develop tolerance to the sedative properties of antihistamines despite continued anticholinergic and antimuscarinic side effects 1

3. Underestimating next-day impairment: The residual sedative effect of nighttime antihistamines has been verified by PET measurement showing significant H₁ receptor occupancy in the brain the next morning 2

4. Assuming subjective alertness equals safety: Performance impairment can exist without the patient feeling drowsy, creating a dangerous situation 1

In conclusion, while taking sedating antihistamines at night may seem logical to minimize daytime drowsiness, the evidence shows that this practice can lead to significant next-day impairment without providing additional therapeutic benefit for the underlying allergic condition.