Although people typically use the word ''antihistamine'' to describe drugs for treating allergies, doctors and scientists use the term to describe a class of drug that opposes the activity of histamine receptors in the body. In this sense of the word, antihistamines are subclassified according to the histamine receptor that they act upon. The two largest classes of antihistamines are H-antihistamines and H-antihistamines. Antihistamines that target the histamine H-receptor are used to treat allergic reactions in the nose (e.g., itching, runny nose, and sneezing) as well as for insomnia. They are sometimes also used to treat motion sickness or vertigo caused by problems with the inner ear. Antihistamines that target the histamine H-receptor are used to treat gastric acid conditions (e.g., peptic ulcers and acid reflux). H-antihistamines work by binding to histamine H receptors in mast cells, smooth muscle, and endothelium in the body as well as in the tuberomammillary nucleus in the brain; H-antihistamines bind to histamine H receptors in the upper gastrointestinal tract, primarily in the stomach.

What's the best answer to this question: is histamine an agonist for h1 and h2 receptors?
yes