Allergies occur as a result of an overactive immune response to a substance that the body sees as foreign. The offending substance, or allergen, can be eaten, breathed, injected or touched. The body's exaggerated response can range from a mildly irritating runny nose to a life-threatening reaction called anaphylaxis. Antihistamines are one type of medication used to combat allergies.
Types of Allergies
People can be allergic to nearly anything, but environmental factors, such as dust, mite feces, animal dander, pollen and mold, are the most common cause of allergies. Milder types of allergic reactions include hay fever and skin rashes. Stomach upset, swollen lips and tongue, hives, low blood pressure and difficulty breathing can signify anaphylaxis. This is a significant, possibly life-threatening, reaction affecting the whole body and requires immediate medical help.
How Antihistamines Work
Antihistamines are used to relieve or prevent allergy symptoms. Antihistamines block the action of histamine in the body. Histamine is released when someone with allergies comes into contact with an allergen. With this exposure, histamine then causes allergy symptoms, such as sneezing, runny nose, watery eyes and itching of the nose and eyes. Histamine may provoke allergy symptoms in the skin, sinuses, airways or digestive system. In more serious cases, histamine can trigger anaphylaxis.
Types of Antihistamines
Antihistamines are generally categorized in two groups, first generation and second generation. First-generation antihistamines tend to cause more sedation and include diphenhydramine, chlorpheniramine, brompheniramine, dimenhydrinate and doxylamine. Second-generation antihistamines, such as cetirizine, loratadine, and fexofenadine, may cause less drowsiness.
Obtaining Antihistamines
Antihistamines may be purchased over the counter or may be prescribed by a doctor. First-generation antihistamines are generally available without a prescription. In recent years, cetirizine and loratadine have also become available over the counter. Often, first-generation antihistamines are combined with other types of medication, including pain-relievers and decongestants.
Antihistamine Side Effects
The most common side effect of antihistamines is drowsiness, but other side effects can include dry mouth, dry eyes and difficulty urinating. People with underlying glaucoma, heart disease, chronic breathing difficulties or thyroid disease should consult a medical provider prior to starting any new antihistamine.
