The second generation antihistamines have no central action, and are used only for treatment of allergic reactions. Cetirizine (Zyrtec) is a piperazine derivative, and has a slight sedative effect.Loratidine (Claritin) and fexofenadine (Allegra) are members of the piperadine class and are essentially non-sedating.The second-generation antihistamines bind only to peripheral HThe first-generation antihistamines may be divided into several chemical classes.The side effect profile, which also determines the uses of the drugs, will vary by chemical class.Generally antihistamines are classified into 2 main groups: Talk to your doctor before you begin taking antihistamines, as they could interact with your other medications or might not be suitable if you have other health conditions.Remember that cough or cold remedies often contain antihistamines, so if you are taking these as well you need to work out your total dosage to make sure you don’t take more than the total recommended amount for any one period.receptor sites, responsible for immediate hypersensitivity reactions such as sneezing and itching.
Promethazine (Phenergan), in contrast, is a phenothiazine, chemically related to the major tranquilizers, and while it is used for treatment of allergies, may also be used as a sedative, the relieve anxiety prior to surgery, as an anti-nauseant, and for control of motion sickness.
Runny nose, sneezing, itchy and watery eyes—the symptoms of allergies can make you miserable and desperate for relief.
Avoiding substances that trigger your allergies can help, but since that is not always possible, you may reach for a medication.
Additional effects vary with the individual drug used.
Several of the older drugs, called first-generation antihistamines, bind non-selectively to H receptors in the central nervous system as well as to peripheral receptors, and can produce sedation, inhibition of nausea and vomiting, and reduction of motion sickness.