Which drug acts as an H1-receptor antagonist and stimulates appetite due to serotonin antagonism?

The role of the drug in question, which acts as an H1-receptor antagonist while also stimulating appetite due to its serotonin antagonism, is fulfilled by cyproheptadine. This drug is known for its dual action: as an antihistamine, it effectively blocks H1 receptors, which helps mitigate allergic symptoms. Furthermore, cyproheptadine also possesses properties that antagonize serotonin receptors, particularly the 5-HT2 and 5-HT3 receptors. This action on serotonin receptors is significant because it can lead to increased appetite, making cyproheptadine a drug of choice in certain conditions where appetite stimulation is desired, such as in patients who are experiencing weight loss or have conditions that result in reduced appetite.

In contrast, other antihistamines listed do not have this stimulating effect on appetite due to serotonin antagonism. Hydroxyzine and loratadine are primarily used for their antihistaminic effects without the appetite-stimulating properties. Diphenhydramine, while also an H1 antagonist, does not exert the serotonin antagonism seen with cyproheptadine and thus lacks the appetite-increasing effects associated with it. Thus, cyproheptadine is uniquely suited for the clinical scenario described.