H1 antagonists, also called H1 blockers, are a class of medications that block the action of histamine at the H1 receptor, helping to relieve allergic reactions. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines.
Diphenhydramine (DPH) is an antihistamine and sedative mainly used to treat allergies, insomnia, and symptoms of the common cold. It is also less commonly used for tremors in parkinsonism, and nausea. It is taken by mouth, injected into a vein, injected into a muscle, or applied to the skin. Maximal effect is typically around two hours after a dose, and effects can last for up to seven hours.
Brompheniramine, sold under the brand name Dimetapp among others, is a first-generation antihistamine drug of the propylamine (alkylamine) class. It is indicated for the treatment of the symptoms of the common cold and allergic rhinitis, such as runny nose, itchy eyes, watery eyes, and sneezing. Like the other first-generation drugs of its class, it is considered a sedating antihistamine.
Chlorphenamine, also known as chlorpheniramine, is an antihistamine used to treat the symptoms of allergic conditions such as allergic rhinitis. It is taken orally. The medication takes effect within two hours and lasts for about 4–6 hours. It is a first-generation antihistamine and works by blocking the H1 receptor.
Hydroxyzine, sold under the brand names Atarax and Vistaril among others, is an antihistamine medication. It is used in the treatment of itchiness, insomnia, anxiety, and nausea, including that due to motion sickness. It is used either by mouth or injection into a muscle.
Cetirizine is a second-generation antihistamine used to treat allergic rhinitis, dermatitis, and urticaria (hives). It is taken by mouth. Effects generally begin within thirty minutes and last for about a day. The degree of benefit is similar to other antihistamines such as diphenhydramine, which is a first-generation antihistamine.
Promethazine, sold under the brand name Phenergan among others, is a first-generation antihistamine, sedative, and antiemetic used to treat allergies, insomnia, and nausea. It may also help with some symptoms associated with the common cold and may also be used for sedating people who are agitated or anxious, an effect that has led to some recreational use. Promethazine is taken by mouth (oral), as a rectal suppository, or by injection into a muscle (IM).
Doxylamine, sold under the brand name Unisom among others, is an antihistamine medication which is used in the treatment of insomnia and allergies. It is also used to treat morning sickness in pregnant women in combination with pyridoxine (vitamin B6). Doxylamine is available over-the-counter, and is used in nighttime cold medicines, such as NyQuil, as well as in pain medications containing acetaminophen and codeine, to help with sleep. The medication is taken by mouth.
Clemastine, also known as meclastin, is a first-generation H1 histamine antagonist (antihistamine) with anticholinergic properties (drying) and sedative side effects. Like all first-generation antihistamines, it is sedating.
Doxepin is a medication belonging to the tricyclic antidepressant (TCA) class of drugs used to treat major depressive disorder, anxiety disorders, chronic hives, and insomnia. For hives it is a less preferred alternative to antihistamines. It has a mild to moderate benefit for sleeping problems. It is used as a cream for itchiness due to atopic dermatitis or lichen simplex chronicus.
Orphenadrine is an anticholinergic drug of the ethanolamine antihistamine class; it is closely related to diphenhydramine. It is a muscle relaxant that is used to treat muscle pain and to help with motor control in Parkinson's disease, but has largely been superseded by newer drugs. It is considered a dirty drug due to its multiple mechanisms of action in different pathways. It was discovered and developed in the 1940s.
Chloropyramine is a classical first-generation antihistamine drug approved in Eastern European countries for the treatment of allergic conjunctivitis, allergic rhinitis, bronchial asthma, and other atopic (allergic) conditions. Related indications for clinical use include angioedema, allergic reactions to insect bites, food and drug allergies, and anaphylactic shock.
Carbinoxamine is an antihistamine and anticholinergic agent. It is used for hay fever, vasomotor rhinitis, mild urticaria, angioedema, dermatographism and allergic conjunctivitis. Carbinoxamine is a histamine antagonist, specifically an H1-antagonist. The maleic acid salt of the levorotatory isomer is sold as the prescription drug rotoxamine.
Diphenylpyraline is a first-generation antihistamine with anticholinergic effects of the diphenylpiperidine class. It is marketed in Europe for the treatment of allergies. DPP has also been found to act as a dopamine reuptake inhibitor and produces hyperactivity in rodents. It has been shown to be useful in the treatment of Parkinsonism.
Mepyramine, also known as pyrilamine, is a first generation antihistamine, targeting the H1 receptor as an inverse agonist. Mepyramine rapidly permeates the brain, often causing drowsiness. It is often sold as a maleate salt, pyrilamine maleate.
A muscarinic receptor antagonist (MRA), also called an antimuscarinic, is a type of anticholinergic agent that blocks the activity of the muscarinic acetylcholine receptor. The muscarinic receptor is a protein involved in the transmission of signals through certain parts of the nervous system, and muscarinic receptor antagonists work to prevent this transmission from occurring. Notably, muscarinic antagonists reduce the activation of the parasympathetic nervous system. The normal function of the parasympathetic system is often summarised as "rest-and-digest", and includes slowing of the heart, an increased rate of digestion, narrowing of the airways, promotion of urination, and sexual arousal. Muscarinic antagonists counter this parasympathetic "rest-and-digest" response, and also work elsewhere in both the central and peripheral nervous systems.
Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic drug that can be bought without a prescription and provides relief from nasal congestion, sneezing, or hives caused by pollen, dust mites, or animal allergy with few side effects. Antihistamines are usually for short-term treatment. Chronic allergies increase the risk of health problems which antihistamines might not treat, including asthma, sinusitis, and lower respiratory tract infection. Consultation of a medical professional is recommended for those who intend to take antihistamines for longer-term use.
Antazoline is a 1st generation antihistamine with anticholinergic properties used to relieve nasal congestion and in eye drops, usually in combination with naphazoline, to relieve the symptoms of allergic conjunctivitis. To treat allergic conjunctivitis, antazoline can be combined in a solution with tetryzoline. The drug is a Histamine H1 receptor antagonist: selectively binding to but not activating the receptor, thereby blocking the actions of endogenous histamine and subsequently leading to the temporary relief of the negative symptoms brought on by histamine.
Quinupramine is a tricyclic antidepressant (TCA) used in Europe for the treatment of depression.
Peripherally selective drugs have their primary mechanism of action outside of the central nervous system (CNS), usually because they are excluded from the CNS by the blood–brain barrier. By being excluded from the CNS, drugs may act on the rest of the body without producing side-effects related to their effects on the brain or spinal cord. For example, most opioids cause sedation when given at a sufficiently high dose, but peripherally selective opioids can act on the rest of the body without entering the brain and are less likely to cause sedation. These peripherally selective opioids can be used as antidiarrheals, for instance loperamide (Imodium).
