Antihistamines

Substances that attenuate or abolish the effects of histamine by reversible binding to histamine receptors.

A distinction is made among the H1 antagonists:

• Sedating_{H1 antagonists}(1st generation):
  • Bilastin
  • Clemastine
  • Dexchlorpheniramine
  • Dimetindene
  • Hydroxyzine
  • Ketotifen
  • Meclozine
  • Promethazine et al.
• Non-sedating_{H1 antagonists}(2nd generation):
  • Azelastine
  • Cetirizine
  • Desloratadine
  • Ebastine
  • Fexofenadine
  • Levocetirizine
  • Loratadine
  • Mequitazine
  • Mizolastine
  • Terfenadine
  • Rupatadine.

Competitive inhibition of_{H1 receptors}, diaminooxydase inhibition, inhibition of potassium leakage from cells. Inhibition of histamine-mediated effects such as vasodilation, increased vascular permeability via inhibition of the release of proinflammatory cytokines such as IL-4, IL-6, Il-8,IL-13 from mast cells and the expression of the adhesion molecule P-selectin from endothelial cells. The spectrum of action of H1 antihistamines thus includes antiallergic and antiphlogistic effects.

• urticaria
• allergic rhinitis and conjunctivitis
• drug allergies
• insect bites
• Quincke's edema
• pruritus
• Serum sickness

Some agents in this drug group have more sedative properties, so they are used as sleeping pills. Diphenhydramine or doxylamine are examples.

Agents that have a stronger anticholinergic effect, such as diphenhydramine and dimenhydrinate (8-chlorotheophylline salt of diphenhydramine), are used as antiemetics for kinetosis.

Sedation (inability to drive! with the H1 antagonists), dry mouth (due to anticholinergic effect), gastrointestinal disorders, central nervous disorders. The photosensitizing potency of the newer non-sedating antihistamines is not proven.

• Does any form of heart disease already exist? If so, an antihistamine with little or no interaction with the cardiac potassium channels should be chosen.
• Is there a co-medication with the following preparations: macrolides, opiates, imidazoles, psychotropic drugs, anti-malaria drugs? If so, an accurate re-evaluation of the medication should be carried out and the combination of QT time prolonging preparations should be done carefully and only if strictly indicated.
• Are there other risk factors such as liver diseases, electrolyte shifts, special potassium-rich diets? If so, treatment of the underlying diseases and strict indication for the prescription of the antihistamine.

• Netzwerk aktuell (2006) Photosensitization through antihistimanics. Medication telegram 37: 60