Are antihistamines safe for patients with Parkinson's disease?

Antihistamines in Parkinson's Disease: Safety Considerations

First-generation antihistamines should be avoided in patients with Parkinson's disease due to their anticholinergic properties which can worsen cognitive function and potentially exacerbate parkinsonian symptoms.

Types of Antihistamines and Their Effects in PD

First-Generation Antihistamines

• Have significant anticholinergic properties that can:

  • Worsen cognitive function
  • Potentially increase risk of future cognitive decline
  • Interfere with central compensation mechanisms
  • Cause sedation (often subjectively unperceived by patients)
  • Lead to performance impairment 1

• Examples include:

  • Diphenhydramine
  • Chlorpheniramine
  • Cyproheptadine

Second-Generation Antihistamines

• Generally preferred if antihistamines are necessary because they:

  • Have minimal anticholinergic effects
  • Cause less sedation at recommended doses
  • Have less impact on cognitive function 1

• Safer options include:

  • Fexofenadine
  • Loratadine
  • Desloratadine 1

Anticholinergic Burden in Parkinson's Disease

Research shows that 31.5-46.3% of PD patients are already taking medications with anticholinergic properties, creating a significant burden 2. Adding first-generation antihistamines would further increase this burden.

The most concerning medication classes with anticholinergic properties commonly used in PD patients include:

• Antiparkinsonian medications (8.2%)
• Antipsychotics (6.4%)
• Urological medications (3.3%) 2

Historical Context and Current Recommendations

While antihistamines were historically used in the treatment of Parkinson's disease before the development of modern antiparkinsonian medications 3, current evidence does not support their routine use in PD management except in specific circumstances:

1. For treatment of comorbid allergic conditions:

   • Second-generation antihistamines should be preferred
   • Monitor for any worsening of PD symptoms or cognitive function

2. For palliative care in end-of-life situations:

   • Parenteral diphenhydramine may be considered for severe tremors in patients who cannot swallow and are near death 4
   • This is a very limited exception for specific palliative scenarios

Potential Risks of Antihistamines in PD

1. Cognitive Impairment: Anticholinergic medications are associated with increased risk of cognitive decline, to which PD patients are already vulnerable 2

2. Drug Interactions: Potential interactions with other medications commonly used in PD management 5

3. Sedation and Falls Risk: Increased sedation may worsen mobility issues and increase fall risk

4. Opposing Mechanisms: Some PD patients are prescribed cholinesterase inhibitors for cognitive symptoms - anticholinergic antihistamines directly oppose their mechanism of action 2

Recommendations for Clinical Practice

1. Avoid first-generation antihistamines in patients with Parkinson's disease whenever possible

2. If antihistamine therapy is necessary (e.g., for allergic rhinitis):

   • Use second-generation agents like fexofenadine, loratadine, or desloratadine
   • Consider alternative non-sedating treatments for allergic conditions
   • For allergic rhinitis, intranasal corticosteroids are more effective than oral antihistamines and have minimal systemic effects 1

3. Monitor patients who require antihistamines for:

   • Changes in cognitive function
   • Worsening of parkinsonian symptoms
   • Increased sedation or fall risk

4. Review medication lists regularly to identify and minimize overall anticholinergic burden in PD patients

By carefully selecting appropriate antihistamines when necessary and considering alternative treatments for allergic conditions, clinicians can minimize the potential negative impacts on Parkinson's disease patients' symptoms and cognitive function.