H1 antagonists, also known as H1 blockers, are a class of medications that primarily treat allergic reactions and conditions like hay fever. They work by blocking the action of histamine, a substance in the body that causes allergy symptoms such as itching, sneezing, and runny nose. Common examples include diphenhydramine and loratadine. These medications can also be used to alleviate symptoms of motion sickness and insomnia due to their sedative effects. H1 antagonists are generally categorized into first-generation, which may cause drowsiness, and second-generation, which are less likely to cause sedation.