Cetirizine and desloratadine are among the many types of antihistamine available without a prescription to treat allergy symptoms, according to MedlinePlus. Antihistamines work by reducing or blocking histamines, chemicals produced by the body when triggered by contact with substances such as pollen or pet dander, explains WebMD. Medications such as Allegra-D, Claritin-D and Zyrtec-D combine an antihistamine with a decongestant to further relieve allergy symptoms.
Medications including Benadryl, Chlor-Trimeton and Tavist belong to "first generation" antihistamines, older forms of antihistamine that often produce unwanted side effects such as blurred vision, drowsiness and dizziness, states WebMD. Newer medications such as Allegra, Clarinex and Zyrtec tend to produce fewer of these side effects.
Although most antihistamines are considered safe for adults and children ages 2 and older, ask a health care provider or pharmacist about safety and dosage issues, interactions with other medications and potential side effects before taking or giving the medication to children, states MedlinePlus. Adults taking antihistamines should know how the medication affects them before driving or operating machinery. Do not drink alcohol while taking antihistamines and consult a physician before taking the medication if there is a history of medical conditions including heart disease, high blood pressure, epilepsy and diabetes.