Effects mediated by H1 histamine receptor include:
Sedation occurs with those drugs that are potent antagonists of the H1-histamine receptor, including chlorpromazine, olanzapine, quetiapine and clozapine.
Histamine H1 receptors mediate many pathological processes, including allergic rhinitis, atopic dermatitis, conjunctivitis, urticaria, bronchoconstriction, asthma and anaphylaxis. Moreover, histamine stimulates the parietal cells in the stomach, causing an increase in acid secretion via the activation of H2 receptors.​
Reference: Lippincott Illustrated Reviews: Pharmacology Wolters Kluwer Sixth Edition page no 150,394​
Inhibition of gastric acid secretion
Induction of hepatic cytochrome P450 enzymes
Maintenance of a wakeful state
Vasoconstriction of arterioles