Recreational drug use are caused by an excessive response of the body to allergens, such as the pollen released by grasses and trees. An allergic reaction causes release of histamine by the body. Other uses of antihistamines are to help with normal symptoms of insect stings even if there is no allergic reaction. Their recreational appeal exists mainly due to their anticholinergic properties, that induce anxiolysis and, in some cases such as diphenhydramine, chlorpheniramine, and orphenadrine, a characteristic euphoria at moderate doses. High dosages taken to induce recreational drug effects may lead to overdoses. Antihistamines are also consumed in combination with alcohol, particularly
Side reaction competing reaction when different compounds (B, C) compete for another reactant (A). If the side reaction occurs about as often as the main reaction, it is spoken of parallel reactions (especially in the kinetics, see below). Also there may be more complicated relationships: Compound A could reversibly but quickly react to substance B (with speed k) or irreversible but slow (k> k » k) to substance C: Assuming that the reaction to substance C is irreversible, as it is thermodynamically very stable. In this case, B is the kinetic and C is the thermodynamic product of the reaction (see also