Antihistamine tablets in their packaging. These are marketed as Centrin 10. Antihistamine drugs counteract the effect of histamine, a chemical made by the body that is responsible for many symptoms of an allergic response. Histamines are produced when the body encounters an allergen that is has been sensitised to, such as pollen in the case of hay fever. Other conditions treated by antihistamines can include reactions to animal stings, such as a bee or wasp sting.