Promethazine
Promethazine is a first-generation antihistamine used for allergies, nausea, motion sickness, and sedation. Its effects include pronounced drowsiness.
Histamine H1 receptor: A receptor that mediates many effects of histamine, including itching, vascular permeability, and wakefulness. Promethazine blocks this receptor, reducing allergy symptoms and promoting drowsiness.
Allergic rhinitis: Inflammation of the nasal lining triggered by allergens, commonly causing sneezing, itching, and congestion. Promethazine can relieve histamine-driven sneezing and itching in allergic rhinitis.
Cetirizine: A second-generation H1 antihistamine used mainly to treat allergic symptoms, usually with less sedation than older antihistamines. Unlike promethazine, cetirizine generally causes less drowsiness and has fewer anticholinergic effects.
Respiratory depression: Inadequate breathing that reduces oxygen intake and may increase carbon dioxide in the blood. Promethazine can worsen respiratory depression, especially with opioids or other central nervous system depressants.
Histamine: A signaling molecule involved in immune responses, stomach acid secretion, and communication in the nervous system. Promethazine counters some effects of histamine by blocking H1 receptors.
Anticholinergic: A substance that inhibits signaling by acetylcholine at muscarinic receptors. Promethazine's anticholinergic action helps suppress nausea but can cause dry mouth and blurred vision.
Motion sickness: Nausea and related symptoms caused by conflicting signals about movement from the senses and vestibular system. Promethazine can prevent motion-sickness symptoms through central antihistamine and anticholinergic effects.
Diphenhydramine: A first-generation H1 antihistamine used for allergies, itching, motion sickness, and short-term sleep aid. Both drugs can be sedating, but their approved uses and risks differ by formulation and setting.
Opioid: A substance that activates opioid receptors to relieve pain and can also cause sedation and respiratory depression. Combining opioids with promethazine can intensify sedation and dangerously suppress breathing.
First-generation antihistamine: An older class of H1 receptor blockers that readily enter the brain and often cause sedation. Promethazine belongs to this class, whose brain penetration distinguishes it from newer antihistamines.