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Most people don't have much trouble with allergy shots. But they contain the substances that cause your allergies — so reactions are possible, and can include:

• Local reactions, which can involve redness, swelling or irritation at the injection site. These common reactions typically begin within a few hours of the injection and clear up soon after.
• Systemic reactions, which are less common — but potentially more serious. You may develop sneezing, nasal congestion or hives. More-severe reactions may include throat swelling, wheezing or chest tightness.
• Anaphylaxis is a rare life-threatening reaction to allergy shots. It can cause low blood pressure and trouble breathing. Anaphylaxis often begins within 30 minutes of the injection, but sometimes starts later than that.

If you get weekly or monthly shots on a regular schedule without missing doses, you're less likely to have a serious reaction.

Taking an antihistamine medication before getting your allergy shot can reduce the risk of a reaction. Check with your doctor to see if this is recommended for you.

The possibility of a severe reaction is scary — but you won't be on your own. You'll be observed in the doctor's office for 30 minutes after each shot, when the most serious reactions usually occur. If you have a severe reaction after you leave, return to your doctor's office or go to the nearest emergency room.

Before starting a course of allergy shots, your doctor may use a skin test to determine that your reactions are caused by an allergy — and which specific allergens cause your signs and symptoms. During a skin test, a small amount of the suspected allergen is scratched into your skin and the area is then observed for about 15 minutes. Swelling and redness indicate an allergy to the substance. Your doctor may also use an allergy blood test.

When you go in for allergy shots, let the nurses or doctors know if you are feeling unwell in any way. This is especially important if you have asthma. Also let them know if you had any symptoms after a previous allergy shot.

Allergy shots are usually injected in the upper arm.

To be effective, allergy shots are given on a schedule that involves two phases:

• The buildup phase generally takes three to six months. Typically shots are given one to three times a week. During the buildup phase, the allergen dose is gradually increased with each shot.
• The maintenance phase generally continues for three to five years or longer. You'll need maintenance shots about once a month.

In some cases the buildup phase is done more quickly (rush immunotherapy), which requires several injections of increasing doses during each doctor visit. This can decrease the amount of time you need to reach the maintenance phase and get relief from allergy symptoms, but it also increases your risk of having a severe reaction.

You will need to remain in the doctor's office for 30 minutes after each shot, in case you have a reaction.

Allergy symptoms won't stop overnight. They usually improve during the first year of treatment, but the most noticeable improvement often happens during the second year. By the third year, most people are desensitized to the allergens contained in the shots — and no longer have significant allergic reactions to those substances.

After a few years of successful treatment, some people don't have significant allergy problems even after allergy shots are stopped. Other people need ongoing shots to keep symptoms under control.