• Routes: Oral, nasal, ophthalmic, parenteral, rectal
• Peak [blood]: within 2-3 hours 
• Widely distributed in body (2nd do not enter CNS = less sedative side effects) 
• Some are extensively metabolized, especially by the liver 
• Duration of 4-6 hours (12-24 for 2nd gen)

Pharmacodynamics of H1 Receptor Antagonists

• Reversible competitive inhibition of histamine receptors
• 1st gen also have effects on muscarinic, α-adrinergic, serotonin and anesthetic receptors
• Some 2nd gen, especially, cetirizine, inhibit histamine release from mast cells

• Antiparkinsonism effects (due to central antimuscarinic actions)
• Anticholinoceptor actions (atropine like effects, inhibition of parasympathetic nervous system)
• Adrenoceptor blocking actions
• Serotinin blocking action (1st gen)
• Local anethesia, in high doses (diphenhydramine, promethazine)

• Inhibition of edema
• Partial inhibition of histamine-induced hypotension
• Inhibition of bronchiolar and GI smooth muscle contraction
• Inhibition of nausea and vomiting
• Sedation (1st gen)

• Allergic reactions
• Motion sickness and vestibular disturbances (may be due to blockade of cholinergic and histaminergic signals from the vestibular apparatus of the inner ear to the vomiting center of the brain)
• Systemic mastocytosis
• Parkinsonism-like problems (diphenhydramine, antimuscarinic actions)

• 1st gen: mild dizziness, fatigue, nausea, constipation,diarrhea, sedation
• Xerostomia (dry mouth), dryness of respiratory passages, urinary retention, dysuria, blurred vision
• Drug allergy
• Excitation and convulsions in children
• Orthostatic hypotension
• Overdose