Promethazine is a phenothiazine derivative with histamine H1-blocking, antimuscarinic, and sedative properties. Promethazine HCl Oral Solution is useful for: perennial and seasonal allergic rhinitis. Allergic conjunctivitis due to inhalant allergens and foods. Anaphylactic reactions, as adjunctive therapy to epinephrine and other standard measures, after the acute manifestations have been controlled. Preoperative, postoperative, or obstetric sedation. Prevention and control of nausea and vomiting associated with certain types of anesthesia and surgery. Therapy adjunctive to meperidine or other analgesics for control of post-operative pain. Active and prophylactic treatment of motion sickness. Antiemetic therapy in postoperative patients.

The alkaloid L-(-)-scopolamine [L-(-)-hyoscine], a belladonna alkaloid, competitively inhibits muscarinic receptors for acetylcholine and acts as a nonselective muscarinic antagonist, producing both peripheral antimuscarinic properties and central sedative, antiemetic, and amnestic effects. Scopolamine acts: i) as a competitive inhibitor at postganglionic muscarinic receptor sites of the parasympathetic nervous system, and ii) on smooth muscles that respond to acetylcholine but lack cholinergic innervation. It has been suggested that scopolamine acts in the central nervous system (CNS) by blocking cholinergic transmission from the vestibular nuclei to higher centers in the CNS and from the reticular formation to the vomiting center. Scopolamine can inhibit the secretion of saliva and sweat, decrease gastrointestinal secretions and motility, cause drowsiness, dilate the pupils, increase heart rate, and depress motor function. Scopolamine is used for premedication in anesthesia and for the prevention of nausea and vomiting (post operative and associated with motion sickness).

Meclizine, a piperazine-derivative H1-receptor antagonist similar to buclizine, cyclizine, and hydroxyzine, is used as an antivertigo/antiemetic agent. Meclizine is used in the management of nausea, vomiting, and dizziness associated with motion sickness and vertigo in diseases affecting the vestibular apparatus. Along with its actions as an antagonist at H1-receptors, meclizine also possesses anticholinergic, central nervous system depressant, and local anesthetic effects. Meclizine depresses labyrinth excitability and vestibular stimulation and may affect the medullary chemoreceptor trigger zone. It is sold under the brand names Bonine, Bonamine, Antivert, Postafen, Sea Legs, and Dramamine II.

Cyclizine (cyclizine hydrochloride, Valoid®) is a histamine H1 antagonist of the piperazine class which is characterised by a low incidence of drowsiness. It possesses anticholinergic and antiemetic properties. The exact mechanism by which cyclizin (cyclizine hydrochloride, Valoid®) can prevent or suppress both nausea and vomiting from various causes is unknown. It increases lower oesophageal sphincter tone and reduces the sensitivity of the labyrinthine apparatus. It may inhibit the part of the midbrain known collectively as the emetic centre.

Pheniramine is an antihistamine used to treat allergic conditions such as hay fever or urticaria. It is generally sold in combination with other medications, rather than as a stand-alone drug. Allergies are caused by an excessive type 1 hypersensitivity response of the body to allergens, mediated by inappropriate histamine signalling. By inhibiting the binding of histamine, antihistamines decrease the normal histamine response from cells, consequently decreasing allergic symptoms. Antihistamines such as pheniramine appear to compete with histamine for histamine H1- receptor sites on effector cells. The antihistamines antagonize those pharmacological effects of histamine which are mediated through activation of H1- receptor sites and thereby reduce the intensity of allergic reactions and tissue injury response involving histamine release. Antihistamines suppress the histamine-induced wheal (swelling) and flare (vasodilation) response by blocking the binding of histamine to its receptors on nerves, vascular smooth muscle, glandular cells, endothelium, and mast cells. They effectively exert competitive antagonism of histamine for H1-receptors. Pheniramine is marketed under the trade name Avil and Visine-A among others).

Buclizine, a piperazine derivative, is a sedating antihistamine with antimuscarinic and moderate sedative action. The drug is used mainly for its antiemetic action, particularly in the prevention of motion sickness, and in the treatment of migraine in combination with analgesics. The following side/adverse effects have been selected on the basis of their potential clinical significance: drowsiness; Incidence less frequent; blurred vision; dryness of mouth, nose, and throat; headache; nervousness, restlessness, or trouble in sleeping; upset stomach. The following drug interactions have been selected on the basis of their potential clinical significance: alcohol; anticholinergics or other medications with anticholinergic activity; apomorphine.

Proroxan is a non-selective а-adrenoblocker. Proroxan was found to prevent the development of hypertensive crises and improve cerebral bioelectrical parameters in most of hypertensive patients. Proroxan has been used as an antihypertensive and in the treatment of Ménière’s disease, motion sickness, and allergic dermatitis.

Flunarizine is a selective calcium entry blocker with calmodulin binding properties and histamine H1 / dopamine D2 blocking activity. It is not available in the US but marketed in other countries for prophylaxis of a migraine, occlusive peripheral vascular disease, the vertigo of central and peripheral origin, motion sickness and as an adjuvant in the therapy of epilepsy. The drug is also investigated for the treatment of schizophrenia.

Cinnarizine is a piperazine derivative with antihistaminic, antiserotonergic, antidopaminergic, and calcium channel-blocking activities. It inhibits calcium translocation across the vestibular sensory cells in the ampullae and maintains endolymph flow by preventing constriction of the stria vascularis. It is currently used for the treatment of nausea, vomiting, and vertigo caused by Meniere’s disease and other vestibular disorders. Cinnarizine is also used for prevention and treatment of motion sickness. Chronic use of cinnarizine may induce extrapyramidal symptoms.

Promethazine is a phenothiazine derivative with histamine H1-blocking, antimuscarinic, and sedative properties. Promethazine HCl Oral Solution is useful for: perennial and seasonal allergic rhinitis. Allergic conjunctivitis due to inhalant allergens and foods. Anaphylactic reactions, as adjunctive therapy to epinephrine and other standard measures, after the acute manifestations have been controlled. Preoperative, postoperative, or obstetric sedation. Prevention and control of nausea and vomiting associated with certain types of anesthesia and surgery. Therapy adjunctive to meperidine or other analgesics for control of post-operative pain. Active and prophylactic treatment of motion sickness. Antiemetic therapy in postoperative patients.