Abirritant
Abirritants, also known as anti-itch drugs are medications used for relieving or preventing itching (pruritus) around the body. Common indications for abirritants include dermatitis, eczema, psoriasis, allergic reactions, itch caused by chronic kidney disease and related conditions. Abirritants consist of a large group of drugs belonging to different classes with varying mechanisms to treat itch. They may work either directly or indirectly to relieve itch, and evidence on their effectiveness varies from one class to another. Some alternative medicines are also used to treat itch. Side effects of abirritants also vary depending on the class of the drug . Even before the emergence of modern evidence-based medicine, abirritants have already been used in many civilizations, but practices and choice of drugs may differ by culture. Types. A number of drug classes are available as abirritants for itching relief, but there is no one single specific abirritant to treat all forms of itch. Treatments may vary depending on the cause. Commonly prescribed abirritants can be broadly divided into topical and systemic drugs, and may include a combination of one or more drugs, described as below: Topical drugs. Topical formulations are preferred for treating localized itch caused by skin damage , inflammation or dryness. Systemic drugs. Generalized itch, or itching across the whole body, can be a symptom of a dermatological disorder or an underlying systemic problem. Some systemic diseases can that cause generalized itch include diabetes, hypothyroidism, kidney diseases and liver diseases. It is usually treated with systemic agents instead of topical agents. Corticosteroids and antihistamines mentioned above can also be used to treat generalized itch. Common systemic abirritants are listed below: Alternative treatments. A number of herbs have been used to treat itching such as cannabis, pigweed (portulaca oleracaea), Ashoka (sarco asoca), and fig (fificus carica). Other unconventional forms  of treatment with potential efficacy for treating systemic itch include topical cannabinoids and H4 antihistamines. Effectiveness. Despite the availability of many forms of treatment, there is only a limited number of case series or small-scale studies examining the efficacy of abirritants. There is a lack of evidence on treatment for chronic pruritus of unknown origin. There is also little to no evidence on the efficacy and safety of using abirritants during pregnancy. Treating itch associated with disease. Some abirritants work by indirectly treating itch through treating the causative medical conditions, which means that the itching associated with the condition  will often subside when it is properly treated. This includes antihistamines and corticosteroids, which are effective in treating inflammatory disorders of the skin, in particular atopic dermatitis. Successful treatment of atopic dermatitis with either corticosteroids or antihistamines would resolve the associated itching. Treating itch directly. Some abirritants treat pruritus directly without necessarily treating the causative medical condition. Abirritants that directly treat itching and are established to be effective are reported here in the table below: Traditional Chinese Medicine. Traditional Chinese medicine is extensively used in Asia for relief of itch. It is believed that itching is caused by irritations from  wind, dampness or blood stasis, and can be relieved by the use of herbs such as chrysanthemum, gardenia fruit or mung bean. Sometimes these herbal remedies are used in combination with acupuncture and moxibustion, but their efficacy is still unclear. Sericin cream and oral omega-3 fatty acid supplements may show benefit in reducing itch. Adverse effects. Each class of abirritants has its own set of potential adverse effects. Systemic Corticosteroids. Systemic corticosteroid use has been associated with a wide range of potential adverse effects. In a review article, the following common complications were noted for prolonged use: redistribution of fat  tissues (moon face), high blood sugar, infections, delayed wound healing, and HPA axis suppression, where the body's natural production of hormones like corticotropin-releasing hormone and adrenocorticotropic hormone is suppressed as a response to the increased level of corticosteroids in the blood. There is a lack of data on adverse effects associated with corticosteroid use of a shorter period and lower dose. Topical Corticosteroids. Both local and systemic side effects can result from topical corticosteroid use, especially in prolonged treatment. Local side effects can occur regularly from prolonged use, which include skin atrophy (thinning), stretch marks, infections, lighter skin  color, and sudden decrease in efficacy of the drug. Systemic side effects are far less prevalent than local ones. Prolonged high potency corticosteroids use on thin skin, especially in children, increases the risk of systemic side effects since thin skin allows for greater absorption. One commonly cited systemic side effect from topical use is HPA axis suppression. A meta-analysis of topical corticosteroid use in children concluded that low-potency corticosteroid at recommended dosages and duration do not cause clinically significant HPA suppression. Antihistamines. Antihistamines target the molecule histamine by blocking the histamine H1 receptor. First-generation antihistamines like diphenhydramine  and chlorpheniramine are able to move from the blood into the brain across the blood–brain barrier, where they block the H1 receptor, reducing the neurotransmitter effect of histamine, leading to central nervous system side effects such as drowsiness and confusion. Second generation antihistamines, such as fexofenadine and cetirizine are less able to move from blood circulation into the brain and are therefore associated with fewer side effects in usual doses. μ-opioid receptor antagonists. μ-opioid receptor antagonists are usually well-tolerated and have no abuse potential since they do not cause physical dependence. Side effects are dose-dependent  and generally limited to the first two weeks of treatment. Opioid withdrawal symptoms are rare and may include severe lightheadedness, depersonalization and anxiety. Antidepressants. Serotonin reuptake inhibitors, including both serotonin-norepinephrine reuptake inhibitors (SNRIs) and selective serotonin reuptake inhibitors (SSRIs), are generally well tolerated. Common side effects include: Immunosuppressants. Immunosuppressants may cause immunodeficiency, resulting in an increased susceptibility to infection. Other side effects include bone marrow suppression, increased risk of cardiovascular disease and increased risk of cancer. Capsaicin. Being the main chemical that causes heat in chili pepper, the main side effect of capsaicin is a burning sensation that usually  persists for several days. A topical anaesthetic can be used to reduce the sensation. In addition, the topical anaesthetic can also provide anti-itch effect on its own. History. Abirritants have an extensive history in treating itch. The history of abirritants dates back to the Byzantine period, when Alexander of Tralleis, a famous physician, recommended crushed rue and alum mixed in honey for topical application to the scalp for itching caused by scabby conditions of the head. During the 7th century, Paul of Aegina, a famous Greek physician, described a list of drugs for treatment of itch including plants such  as the squill, metallic components, and goat droppings which were applied externally. These drugs are common in ancient pharmacopeia. The "Lorscher Azneibuch" written in the monastery of Lorsch in the 7th century described many preparations of abirritants for both systemic and topical use, such as an ointment prepared from stinging nettle seeds. Mercury-coated girdles were used in the 17th century as an expensive treatment to alleviate symptoms of itch caused by scabies, but mercury toxins in the blood often caused other troubling symptoms in patients. In the 20th century, many new abirritants for external use emerged, including salicylic acids , naphthol, tar, carbolic acid, thymol, and menthol, which were mostly available in the form of ointments. Alcohol and opium were also commonly prescribed.