Skip to main content

Antihistamine Side Effects: What to Know About Common and Serious Effects

Last reviewed: December 2025

Overview

Antihistamine side effects vary significantly between first-generation (sedating) and second-generation (non-sedating) types. Understanding these differences helps in choosing the right antihistamine and recognizing when side effects require attention.

While antihistamines are generally safe for mild allergic symptoms, they are not appropriate for severe allergic reactions. Side effects from antihistamines should not be confused with worsening allergic symptoms that require emergency care.

First-Generation Antihistamine Side Effects

First-generation antihistamines (diphenhydramine, chlorpheniramine, clemastine) cross the blood-brain barrier and affect the central nervous system, causing significant side effects.

Common Side Effects

Central nervous system effects:

  • Drowsiness (very common)
  • Dizziness
  • Impaired coordination
  • Slowed reaction time
  • Difficulty concentrating
  • In children: paradoxical excitability or hyperactivity

Anticholinergic effects:

  • Dry mouth
  • Dry eyes
  • Blurred vision
  • Constipation
  • Urinary retention (difficulty urinating)
  • Increased heart rate

Serious Concerns

Cognitive impairment in elderly: First-generation antihistamines are included on the Beers Criteria list of medications to avoid in older adults. They can cause:

  • Confusion
  • Memory problems
  • Increased fall risk
  • Delirium

Impaired driving: Studies show first-generation antihistamines impair driving ability similar to alcohol intoxication. The FDA and NHTSA warn against driving or operating machinery after taking these medications.

Urinary retention: Men with enlarged prostate are at particular risk for urinary retention, which can become a medical emergency.

Second-Generation Antihistamine Side Effects

Second-generation antihistamines (cetirizine, loratadine, fexofenadine, levocetirizine) were designed to minimize crossing into the brain, resulting in fewer side effects.

Common Side Effects

Generally mild:

  • Headache
  • Dry mouth (less common than first-generation)
  • Fatigue (more common with cetirizine)
  • Nausea (uncommon)

Cetirizine-specific:

  • More likely to cause drowsiness than other second-generation options
  • Fatigue reported by some users

Fexofenadine-specific:

  • Least likely to cause any sedation
  • Occasional headache
  • Nausea with higher doses

Loratadine-specific:

  • Rarely causes drowsiness
  • Headache in some users
  • Dry mouth (uncommon)

Why Second-Generation Is Preferred

Second-generation antihistamines:

  • Cause little to no drowsiness (except cetirizine in some people)
  • Do not significantly impair driving
  • Have fewer anticholinergic effects
  • Are safer for elderly patients
  • Can be taken during the day without affecting work or school

Rare but Serious Side Effects

For all antihistamines, seek medical attention if you experience:

  • Fast or irregular heartbeat
  • Severe dizziness or fainting
  • Difficulty urinating (complete inability)
  • Severe allergic reaction to the antihistamine itself (rare):
    • Rash or hives
    • Swelling of face, lips, or tongue
    • Difficulty breathing

Side Effects vs. Worsening Allergic Reaction

It’s important to distinguish antihistamine side effects from signs that your allergic reaction is getting worse:

Antihistamine side effects (not emergencies):

  • Drowsiness
  • Dry mouth
  • Mild headache
  • Constipation

Signs your allergic reaction needs emergency care:

  • Difficulty breathing or wheezing
  • Throat tightness or swelling
  • Dizziness or feeling faint
  • Rapid heartbeat with other symptoms
  • Nausea/vomiting with other symptoms
  • Symptoms spreading or worsening

If symptoms suggest anaphylaxis, antihistamines cannot help—use epinephrine and call 911 immediately. According to the AAAAI, antihistamines never take the place of epinephrine for severe reactions.

Drug Interactions Affecting Side Effects

Increased Sedation Risk

Combining antihistamines (especially first-generation) with these substances increases drowsiness:

  • Alcohol
  • Sleep medications
  • Opioid pain medications
  • Muscle relaxants
  • Anti-anxiety medications (benzodiazepines)
  • Some antidepressants

Anticholinergic Burden

Taking first-generation antihistamines with other anticholinergic medications increases side effect risk:

  • Tricyclic antidepressants
  • Some antipsychotics
  • Some bladder medications
  • Some Parkinson’s medications

This combination is particularly dangerous in elderly patients.

Managing Side Effects

If Drowsiness Is a Problem

  • Switch to a second-generation antihistamine
  • Take first-generation antihistamines only at bedtime
  • Try fexofenadine (least sedating option)
  • Allow time for tolerance to develop (drowsiness may decrease with regular use)

If Dry Mouth Occurs

  • Stay hydrated
  • Use sugar-free gum or lozenges
  • Consider switching to a second-generation antihistamine

If Side Effects Persist

Consult a healthcare provider if side effects:

  • Interfere with daily activities
  • Don’t improve with time
  • Are severe or concerning

Alternative antihistamines or other allergy treatments (nasal corticosteroids, for example) may be more appropriate.

Special Populations

Pregnancy and Breastfeeding

  • Second-generation antihistamines (especially loratadine and cetirizine) are generally preferred during pregnancy
  • First-generation antihistamines may affect the newborn if used near delivery
  • Consult a healthcare provider before using any antihistamine during pregnancy or breastfeeding

Children

  • Second-generation antihistamines are preferred for children
  • Paradoxical excitability can occur with first-generation types
  • Always use appropriate pediatric formulations and doses

Elderly

  • Avoid first-generation antihistamines when possible
  • Use lowest effective doses of second-generation antihistamines
  • Monitor for confusion or balance problems

Sources

Last reviewed: December 2025