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Medications that inhibit itching
Antipruritics, abirritants, or anti itch drugs, are medications that inhibit the itching (Latin: pruritus) often associated with sunburns, allergic reactions, eczema, psoriasis, chickenpox, fungal infections, insect bites and stings like those from mosquitoes, fleas, and mites, and contact dermatitis and urticaria caused by plants such as poison ivy (urushiol induced contact dermatitis) or stinging nettle. It can also be 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.

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.

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 also little to no evidence on the efficacy and safety of using abirritants during pregnancy.

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.

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. Opioid withdrawal symptoms are rare and may include severe lightheadedness, depersonalization and anxiety. Common side effects include:
Sexual dysfunction, which is the most common side effect of all serotonin reuptake inhibitors, manifesting as delayed ejaculation, inability to achieve orgasm and decreased libido. In response, the United States Food and Drug Administration issued a warning in 2004 regarding the elevated risk of suicidal thoughts and behavior in children and adolescents.

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.

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. 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.