H1 antihistamines are a class of medications that block the action of histamine, a substance in the body that causes allergic symptoms. They are commonly used to treat conditions like allergic rhinitis, hay fever, and hives. By preventing histamine from binding to its receptors, these drugs help reduce symptoms such as sneezing, itching, and runny nose. There are two types of H1 antihistamines: first-generation and second-generation. First-generation H1 antihistamines, like diphenhydramine, can cause drowsiness, while second-generation options, such as loratadine, are less likely to cause sedation. Both types are effective in managing allergy symptoms.