Histamine is is released as a result of allergies or physical trauma. Antihistamines have drying, sedative, and anti-itch effects. These agents are most effective for allergic rhinitis and are commonly included in cold preparations to help relieve runny nose, itching, watery eyes and to decrease mucous production.

Common Antihistamines found in Over-the-Counter Cold Remedies Diphenhydramine Brompheniramine Chlorpheniramine Phenyltoloxamine Pyrilamine

Drawbacks to antihistamine use are mostly attributed to their highly sedative effects. The elderly are particularly sensitive to the side effects. A patient with narrow angle glaucoma should not take a cold medication that contains an antihistamine because the drug can counteract the effect of glaucoma agents and thereby increasing pressure in the eye. A patient with benign prostatic hypertrophy (enlarged prostate gland) can experience urinary retention as a side effect of the medication.

Many antihistamines are also available by prescription only. Promethazine (Phenergan™) should be avoided due to side effects. Newer non-sedating agents such as loratidine (Claritin™), fexofenadine (Allegra™) and cetirizine (Zyrtec™) have been developed and are indicated for seasonal allergies. Advantages include less drowsiness, lower anticholinergic activity and less frequent dosing. These may be more suitable for the elderly.