A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these drugs work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.

Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine released by mast cells. They may be used after symptoms appear, though they seem to prove more effective when used preventively. A wide variety of antihistamines are available.

Decongestants constrict blood vessels to counteract the effects of histamine. Nasal sprays and oral systemic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, and agitation.

Use of nasal sprays for longer than several days can cause loss of effectiveness and produce rebound congestion, in which nasal passages become more severely swollen than before treatment.

Topical corticosteroids reduce mucous membrane and skin inflammation and are available by prescription. Allergies tend to become worse as the season progresses and topical corticosteroids are especially effective at reducing this seasonal sensitization.

As a result, they are best started before allergy season begins. Studies have shown that steroid nasal sprays work better for seasonal allergies on an as-needed basis than do antihistamines. Side effects are usually mild but may include headaches, nosebleeds, and unpleasant taste sensations.

Cromolyn sodium (Nasalcrom) prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. Cromolyn sodium is available as a nasal spray and aerosol (a suspension of particles in gas).

Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, bronchodilators, which cause the smooth muscle lining the airways to open, can be very effective. Bronchodilators include adrenaline, albuterol, and theophylline. Other drugs, including steroids, are used to prevent and control asthma attacks.

Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body. Injections involve gradually increasing amounts of allergen, over several weeks or months, with periodic boosters.

Full benefits may take up to several years to achieve and are not seen at all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.

Researchers have developed a number of treatments for allergies that employ new approaches to the problem. One class of new medications is the antileukotrienes (also known as leukotriene modifiers). Some members of this class are montelukast (Singulair), zafirlukast (Accolate), and zileuton (Zyflo).

These drugs block the action of a group of compounds known as the leukotrienes, which contribute to the development of inflammatory reactions. A second category of new drugs is the IgE modifiers, which interfere with the action of mast cells in producing allergic reactions.

The first IgE modifier to be approved for use in the United States by the Food and Drug Administration (FDA) was omalizumab (Xolair), approved in 2003. A third class of antiallergic medications is a group of immunomodulatory medications, topical ointments that interfere with cell mechanisms producing inflammatory responses. Examples of immunomodulatory medications are pimecrolimus (Elidel cream) and tacrolimus (Protopic ointment).

Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life-threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.

By determining which allergens are causing the reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens. Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure. Vitamin C may prevent allergy symptoms. Cromolyn sodium can be used for allergy prevention.