Step into the allergy remedy aisle at the drugstore or supermarket, and you’ll find shelf after shelf of pills, tablets, liquids, and sprays. All promise relief from a variety of allergy symptoms. But which product is right for you?

These answers to common questions can help you narrow your search.

Q. What’s the difference between an antihistamine and a decongestant—and do I need a medicine with both?
A. A product containing both an antihistamine and a decongestant may be helpful if you have multiple allergy symptoms that could benefit from both medicines. Antihistamines help relieve sneezing, itchiness in the eyes and nose, and a runny nose. They also help clear up itchy skin rashes. Decongestants, on the other hand, help ease a stuffed-up nose and sinus pressure caused by swollen nasal passages. Decongestants are available in medicines you take by mouth and nasal sprays.

Q. Are nonprescription allergy products different from prescription allergy medicines?
A. In recent years, some allergy medicines that were once available only by prescription—such as Claritin and Zyrtec—became available on store shelves in their original prescription strength. There are still allergy medicines that require a doctor’s prescription. In general, prescription antihistamines are less apt to cause side effects such as sleepiness, blurred vision, or dry mouth.

Q. Is it necessary to take medicine for my allergies?
A. If your symptoms are mild and occasional, you can try skipping medicine and just avoiding what triggers your allergies. If allergies are an ongoing problem, however, it’s best to use medicine. If left untreated, recurring allergies can lead to other problems, such as ear infections, frequent sore throats, coughing, tiredness, headaches, or disrupted sleep.

When should I take allergy medicine?
A. Don’t wait for a bad allergy flare-up before taking medicine. Take an antihistamine before entering a situation that could irritate your allergies—such as an outdoor outing on a windy day. This way, you can build up levels of the medicine in your body, which makes an allergy attack less severe and more manageable. If your symptoms come on unexpectedly, a dose of a short-acting antihistamine (one that lasts four to six hours) gives faster relief than taking a long-acting medicine that lasts 12 or 24 hours.

Q. Are over-the-counter allergy medicines safe for everyone to take?
A. Not always. Some decongestants may pose problems if you have high blood pressure, heart disease, thyroid problems, diabetes, or an enlarged prostate. Certain antihistamines should also be avoided if you have glaucoma, chronic bronchitis, or emphysema. Always read the warning labels on medicines before taking a dose. And check with your pharmacist or doctor to see if it is safe to mix an allergy medicine with other drugs you may be taking.