When the immune system observes a foreign object such as virus, bacteria or even pollen, dander, mold, etc., it starts releasing histamine.Histamine further binds with the special receptor H1 which sends signals to the brain.Some people deliberately choose to abuse antihistamines, often as a substitute when they can't get their drug of choice.Others may start out using an antihistamine as a sleep aid but find they need higher and higher doses to fall asleep.
Some people use antihistamines such as Phenergan and Vallergan as sedatives for their children, for example when they are travelling on a plane, but this can backfire as some children respond by becoming significantly more active (not a nice thing at 30,000 feet).
Non-sedating antihistamines should be preferentially used whenever possible as most antihistamines are equally efficacious, while adverse effects of sedating antihistamines can be serious.
This review summarizes the pharmacological properties of clinically useful non-sedating antihistamines from the perspective of histamine function in the CNS.
This histamine can then bind with receptors to trigger increased blood flow to the surrounding area, which can lead to symptoms such as swelling and increased secretions, resulting in a blocked or runny nose, watery eyes and, most importantly, itchiness.
Antihistamines don’t stop allergic reactions from happening, but they do block the histamine receptors from being able to be triggered by the histamine that is released, reducing your symptoms.