Antihistamines are a class of medications used to counteract the effects of histamine, a chemical released during an allergic reaction that causes sneezing, a runny nose and itching. They are most frequently used to relieve the symptoms associated with allergies.

Antihistamines may also be used to relieve or prevent symptoms associated with colds and the flu. Some experts have disputed the effect of antihistamines in effectively treating cold symptoms. However, many others maintain that antihistamines are effective in treating certain cold- or flu-related symptoms, including congestion, itchy eyes, a runny nose and sneezing.

Antihistamines work by blocking the histamines released by body cells. Histamines attach to cells in the body and stimulate them, causing symptoms such as itching, sneezing, a runny nose and watery eyes. Antihistamines have a molecular structure that resembles that of histamine, so they attach to the cell receptors that would otherwise accept histamines. With the effects of the histamines interrupted, the severity of symptoms is reduced.

Antihistamines may likewise interrupt the development or severity of cold and flu symptoms, although this is disputed by some experts. They point out that antihistamines work by blocking the effects of histamines, which are not involved in the symptoms of colds or the flu. They also maintain that even if antihistamines are effective in reducing symptoms of these viral infections, antihistamines are likely to block the flow of mucus from the nose, thus actually trapping the cold or flu virus in the body.

Other experts maintain that antihistamines are an effective way to relieve symptoms such as a runny nose associated with colds and the flu. Many cold and flu medications contain antihistamines. Patients are urged to consult with a physician as to whether or not they should use a cold or flu medication that contains antihistamines.