2.1. Irritants
Drugs belonging to this group have stimulant action on the sensory nerve endings of the skin and mucous membranes.
Mustard paper (mustard plaster, poultice) is covered with a thin layer of fat-free mustard. Its irritant action is associated with mustard essential (volatile) oil (it is allyl isothiocyanate; Fig. 2.1). Mustard contains sinigrin glycoside and myrosin enzyme. When a mustard plaster is moistened with warm water (not higher that 40 °С) myrosin causes enzymatic degradation of sinigrin which leads to the formation of mustard essential oil that has a marked irritant action. Mustard plasters are mostly used to treat respiratory disorders, as well as neuralgia and myalgia.
Refined terpentine oil is an essential oil and is obtained by distillation of terpentine from pine-tree (Pinus sylvestris). The main active agent is α-pinene (from the group of terpenes - derivatives of partially or fully hydrated benzene hydrocarbon that is a part of most essential oils) that is quite lipophilic and thus it penetrates through the epidermis, irritating the sensory nerve endings. Drugs containing refined terpentine oil are mainly used locally as balms to treat neuralgias, myalgias and joint pain.