The two major classes of over-the-counter (OTC) medications for treating allergies are antihistamines and decongestants.

Antihistamines help relieve sneezing, itching and runny nose. They work best if you take them routinely during the allergy season rather than waiting until you feel miserable.

Most antihistamines cause few side effects. About one in five people do become drowsy, though, which affects mental alertness. This often becomes less of a problem with continued use. Other side effects include dry mouth, constipation, blurred vision and difficulty urinating. These effects usually occur with higher doses. Children may have nightmares and be nervous, restless and irritable.

There are three classes of antihistamines:

• Alkylamines, including pheniramine maleate (Triaminic); brompheniramine maleate (Bromfed, Dimetane, Dimetapp); chlorpheniramine maleate (Atrohist, Chlor-Trimeton, etc.); and triprolidine hydrochloride (Actifed).
• Ethanolamines, including clemastine fumarate (Tavist) and diphenhydramine hydrochloride (Benadryl).
• Ethylenediamines, including pyrilamine maleate (Triaminic).

Each class works to block the affect of histamine, a substance that the body makes during an allergic reaction. The ethanolamines cause the most drowsiness, and the alkylamines are least sedating. Try one of each class to find out which works best for you.

If you become drowsy when you take OTC antihistamines, you may find that taking a single large dose at bedtime will provide enough relief of symptoms. The drowsy side effect will occur while you sleep.

Another antihistamine, which doesn't fit into these classes, is phenindamine tartrate (Nolahist). It offers another choice for people with allergies.

Long-acting, 8- to 12-hour antihistamines give longer relief and can help you get through the night with fewer allergy symptoms. If you need prompt relief, the 4-hour type begins working faster, often within 20 minutes.

When antihistamines work well, they typically reduce symptoms by 50 percent to 80 percent, but they rarely relieve all symptoms.

Do not take antihistamines if you have angle-closure glaucoma. They can raise your eye pressure. Do not use them if you have trouble urinating because of prostate problems. Avoid them if you have emphysema or chronic bronchitis. The medications may dry the mucus in your chest and cause breathing problems. It is safer not to take them along with antidepressants, tranquilizers, sleeping pills or alcohol.

All OTC antihistamines may make you drowsy, or there may be other reasons not to take them. Your work may require clear thinking and alertness, such as when using dangerous machinery or driving vehicles. In that case, talk to your doctor about the newer, nonsedating antihistamines (astemazole-Hismanal, loratadine-Claritin, fexofenadine-Allegra, and cetirizine-Zyrtec. To purchase them, you need a doctor's prescription.

Decongestants come as topical eye and nose drops and sprays and as oral tablets and liquid. They narrow blood vessels and reduce blood flow in the affected area, which helps clear congestion and improve breathing.

The topical products are applied to the surfaces of the nose or eyes. Oral products work systemically. They have a slower onset of action, but they usually last longer than many topical products.

The nose drops and sprays should be used for no more than three days to avoid rebound swelling in the nose. Congestion increases and can become difficult to treat.

Eye drops are safer, but those that can be bought over the counter do not contain antihistamines. They are much less effective than prescription combination antihistamine/decongestant eye drops.

The oral decongestants relieve stuffy nose and drainage, but do nothing for itching and sneezing. They sometimes cause headache, nervousness, tremors, sleeplessness, fast heartbeat and rapid pulse. They should not be used by people who have high blood pressure, heart problems, thyroid disease, diabetes or prostate problems unless they are under a doctor's care. Because the drugs can interact with some other medications, consult your doctor before taking them with other medications.

Antihistamine/decongestant combinations are best for nasal and eye allergies. The combination products may also reduce the sedating effects of antihistamine taken alone. Beware of combinations that also contain a pain reliever, such as acetaminophen or aspirin. Chronic use can lead to an inflamed liver and bleeding from the stomach or intestine.

An antiinflammatory medication, Cromolyn Sodium Nasalcrom, is now available over the counter. It is a nasal spray that works by preventing mast cells from releasing histamine and other triggers of inflammation. It is effective in treating watery eyes, runny nose and sneezing.

Cromolyn sodium does not stop inflammation once it has begun. For it to help, it should be started before the allergy season begins and used daily during the season. There are few side effects, although some people get nasal stinging or burning.

It is usually okay to treat nasal and eye allergies with OTC medications. Sometimes, though, problems develop such as repeated sinus infections, ear infections, headaches, cough and wheezing or difficulty with exercise. Call your doctor immediately to discuss these problems and how to manage them.

The doctor may prescribe medications that may make you feel better or suggest you see an allergy specialist. Specialists often can stop the allergic condition from getting worse and can reverse the problems.

OTC medications also are available to treat asthma. These include epinephrine inhalers and oral theophylline-ephedrine combinations. People with asthma should not direct their own treatment, though. If OTC medications are wrongly used, they could lead to higher blood pressure, stroke, heart attack or convulsions.

Many advances have been made in treating asthma that can reduce hospitalizations and death rates. Doctor care and a proven treatment program could save your life.

Tips for OTC Medication Use

• Do not drive or use machinery that requires mental alertness when you first take antihistamines. They often cause drowsiness.
• Check with your doctor before taking OTC medications with other prescription drugs. They may interact.
• Do not use out-of-date medication. Check the expiration date on the label. If out of date, flush the medication down the toilet.
• Do not keep medication in the bathroom. The supply will keep its full strength longer when stored in a cool, dry spot with no direct light.
• Avoid storing the product in temperatures that are very hot or very cold.
• Keep all medication out of reach of children- preferably in bottles with a safety-lock top.