Histamine H1 antagonists, commonly known as antihistamines, are medications that block the action of histamine at the H1 receptor sites. They are primarily used to relieve symptoms of allergies, such as sneezing, itching, and runny nose. These drugs can also help with motion sickness and insomnia due to their sedative effects. There are two main types of H1 antagonists: first-generation and second-generation. First-generation antihistamines, like diphenhydramine, can cause drowsiness, while second-generation antihistamines, such as loratadine, are less likely to cause sedation. Both types are effective in managing allergic reactions and related symptoms.