Allergy treatment usually starts with avoiding the substances (allergens) that cause your signs and symptoms. If you can minimize your exposure to allergens — which may include everything from pollen, mold, pet dander and dust mites to certain foods, drugs and chemicals — you'll have less sneezing, coughing and itching.

But because you can't always avoid everything that triggers your allergies, your doctor may prescribe allergy medication. The right medication or combination of medications depends on the allergy symptoms you have. Allergy medications are available in pill, liquid, nasal spray, eyedrop and topical (applied to the skin) forms, some over-the-counter and others by prescription only. To make the best choice, get advice from your doctor, and find out which medications are best for different symptoms.

The main types of allergy medications are:

• Corticosteroids. These medications help prevent and treat the inflammation associated with allergic conditions.
• Antihistamines. These drugs block histamine, an inflammatory chemical released by your immune system during an allergic reaction.
• Decongestants. These drugs relieve nasal and sinus congestion.
• Leukotriene modifiers. These medications block the effects of leukotrienes, inflammatory chemicals released by your immune system during an allergic reaction.
• Mast cell stabilizers. These preparations prevent the release of histamine.

Corticosteroids help prevent and treat the inflammation associated with most allergic conditions, although the site and severity of inflammation varies. Except for some over-the-counter skin creams, corticosteroid medications usually are available only by prescription. They include:

• Nasal sprays. Corticosteroid medications sprayed in the nostrils are the preferred treatment for hay fever because they help prevent and relieve nasal stuffiness, sneezing and an itchy, runny nose. Examples include budesonide (Rhinocort), mometasone (Nasonex), fluticasone (Flonase) and triamcinolone (Nasacort). Although these medications aren't usually immediately effective, you may start to notice improvement after you've used them regularly for days or even a week or two. Nasal corticosteroids are generally safe for extended use. Mild side effects may include an unpleasant smell or taste, or irritation, crusting and bleeding in your nose. Nasal irritation may be especially noticeable during the winter. Rarely, more serious side effects can include sinus damage and infection. Unlike steroids taken by mouth or inhaled deeply through an inhaler or nebulizer, most nasal steroids don't appear to reduce bone density or affect growth in children. Still, to be on the safe side, doctors usually prescribe the lowest effective dose of nasal corticosteroids.
• Eyedrops. Corticosteroid drops formulated for the eyes relieve the redness, tearing and itching caused by hay fever and allergic conjunctivitis. Examples include dexamethasone (Decadron, Dexair, others), fluorometholone (Eflone, Fluor-Op, others) and prednisolone (AK-Pred, Econopred, others). They shouldn't be used if you have glaucoma or an eye infection. They can cause side effects such as blurred vision. Because they've been shown to cause birth defects in animals, you may be advised to avoid them if you are pregnant or plan to become pregnant. Also, if you wear contact lenses, corticosteroid eyedrops increase your risk of eye infections, so you may be advised to switch to eye glasses during treatment.
• Skin creams. Best for relieving the scaling and itching caused by eczema (atopic dermatitis), corticosteroid skin creams come in different strengths. Low-potency skin creams include hydrocortisone (Allercort, Dermacort, others). Medium to very high potency skin creams include triamcinolone (Aristocort, Flutex, others). Although such skin creams are usually safe, they can sometimes cause skin irritation and discoloration.
• Pills, liquids. Oral corticosteroids such as prednisone (Cordrol, Dexasone, others) are sometimes used to treat severe allergy symptoms. Because the long-term use of such medications can cause severe side effects such as cataracts, osteoporosis and muscle weakness, they're usually prescribed only for short periods of time.

Antihistamines block the action of histamine, an inflammatory chemical released by your immune system during an allergic reaction. Blocking histamine reduces such symptoms as redness, swelling, runny nose, itchy, watery eyes, and hives (urticaria). Prescription and over-the-counter antihistamines include:

• Pills, liquids. Over-the-counter oral antihistamines include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton) and clemastine (Tavist). Because these older, first-generation antihistamines may make you sleepy, avoid using them before driving or operating heavy machinery. Newer, second-generation antihistamines — such as loratadine (Claritin), which is available over-the-counter — are less likely to cause sedation. Fexofenadine (Allegra) is a nonsedating prescription antihistamine. Another prescription antihistamine, cetirizine (Zyrtec), has an intermediate risk of causing drowsiness or driving impairment.
• Nasal sprays. The prescription antihistamine azelastine (Astelin) is effective for hay fever, but may cause drowsiness.
• Eyedrops. Prescription eyedrops include emedastine (Emadine), levocabastine (Livostin) and olopatadine (Patanol). Side effects may include redness, tearing, headache and mild stinging or burning. Antihistamine eyedrops increase the risk of eye inflammation for contact lens wearers, so you're safer wearing glasses during treatment.

Decongestants relieve nasal and sinus congestion caused by hay fever, as well as eye congestion caused by allergic conjunctivitis. Usually available over-the-counter, they include:

• Pills, liquids. Many decongestants contain pseudoephedrine (Sudafed, Actifed, others), sometimes in combination with another drug. Medications such as Claritin-D, for example, combine pseudoephedrine with an antihistamine. Because oral decongestants elevate blood pressure, you should avoid them if you have high blood pressure (hypertension). Oral decongestants can also exacerbate the symptoms of prostate enlargement, making urination more difficult.
• Nasal sprays. Examples include phenylephrine (Neo-Synephrine) and oxymetazoline (Afrin). Don't use a decongestant nasal spray for more than two or three days at a time because, after longer use, you may develop severe congestion as soon as you stop.
• Eyedrops. Examples include tetrahydrozoline hydrochloride (Visine). Although these eyedrops are generally safe, your eyes may become persistently red if you overuse them.

These drugs block the effects of leukotrienes, inflammatory chemicals released by your immune system during an allergic reaction. Such medications have proved most effective in treating allergic asthma, but they also relieve hay fever. Leukotriene modifiers are only available by prescription. They're produced in pill and chewable tablet form. Examples include montelukast (Singulair), zileuton (Zyflo) and zafirlukast (Accolate). Headache is the most common side effect of montelukast, and nausea or upset stomach is the most common side effect of zileuton. Headache and nausea are sometimes side effects of zafirlukast.

Mast cell stabilizers prevent the release of histamine, the same inflammatory chemical that antihistamines stop from binding to cells in the mucous membranes. Mast cell stabilizers may also reduce inflammation associated with hay fever and allergic conjunctivitis. They include:

• Nasal spray. Available over-the-counter, the nasal spray cromolyn sodium (NasalCrom, Children's NasalCrom) has no serious side effects but may make the nasal passageways sting and burn, causing increased sneezing. Cromolyn sodium works best when you take it before your symptoms develop. Some people need to use the spray three or four times a day.
• Eyedrops. Several different mast cell stabilizer eyedrops are available by prescription, but none are sold over-the-counter. Cromolyn sodium (Crolom) is available in eyedrop form, as are slightly different mast cell inhibitors, including lodoxamide (Alomide), pemirolast (Alamast) and nedocromil (Alocril). Cromolyn sodium and lodoxamide may make the eyes burn and sting, while pemirolast may cause chills, coughing, sneezing and sore throat. Nedocromil may cause blurred vision or dry, itchy eyes.

If you are taking any other medications or you have a chronic health condition, talk to your doctor or pharmacist before starting any treatment for allergies, to be sure you're not at risk of a drug interaction or other adverse effect.

If you have the type of asthma triggered by allergies, your doctor may recommend medications such as inhaled corticosteroids, inhaled mast cell stabilizers, bronchodilators and allergy desensitization shots (immunotherapy). Omalizumab (Xolair), an anti-IgE monoclonal antibody, may help you if you have severe allergic asthma. Administered by injection every two to four weeks, omalizumab blocks the IgE antibodies that your immune system produces to attack substances that generally cause no harm, such as pollen, dust mites and pet dander.

If you have hay fever symptoms that don't improve with medications or if you aren't able to take allergy medications without side effects, your doctor may recommend allergy shots (immunotherapy). Over a period of three to five years, you receive regular injections containing allergen extracts. The goal is to desensitize you to specific allergens and decrease or eliminate your need for medications.

Immunotherapy may be especially effective if you're allergic to cat dander, dust mites or pollen produced by trees, grass and weeds. In children, immunotherapy may help prevent the development of asthma. Rarely, immunotherapy injections can cause a severe allergic reaction (anaphylaxis).

If you're highly allergic to certain foods, such as peanuts, or to bee or wasp venom, you may be at risk of anaphylactic shock — a sudden, life-threatening allergic reaction. To be on the safe side, your doctor may suggest that you carry an injectable dose of epinephrine (adrenaline). Epinephrine can help slow the reaction while you seek emergency medical treatment. You may be able to administer the drug by yourself, after being taught how to use a self-injecting syringe and needle. A friend, family member or medical professional called in response to a severe anaphylactic reaction also may administer the medication.

Allergy medications vary as much as allergies themselves. By understanding how such medications work, you can help your doctor develop a treatment plan that's best for you.