What Are Antihistamines?

Different medications can treat allergies, including steroids and allergy shots, but one of the first things to try is an antihistamine. These are over-the-counter (OTC) and prescription medications that block histamine, a chemical that causes your body to develop allergic symptoms such as sneezing, itching, a runny nose, and hives.

For lots of people, allergy treatment is reactive. You get stuffed up, your eyes water, and then you go to the medicine cabinet for relief. Most OTC and prescription antihistamines begin to work within 30 minutes and reach peak performance levels after one to two hours. But many doctors say we should be taking the medicine before we have symptoms. Call it allergy pretreatment.

By waiting, you could be risking more than mild discomfort. Once your allergy symptoms start, you might need more heavy-duty medicines to get them under control.

It's a good idea to take your allergy medicines before you need them. If you wait until you have symptoms, they'll probably be harder to treat. (Photo Credit: OJO Images/Getty Images)

What is allergy pretreatment?

It’s simple: Start taking your medicine two to four weeks before the allergy season starts. If you know what triggers your allergies, getting a jump on them should be easy. Remember to continue to take your antihistamines every day, even if you feel fine. Once you stop, your allergy symptoms will most likely return.

When your body comes into contact with any allergy trigger, such as pollen, ragweed, pet dander, or dust mites, it makes chemicals called histamines. They cause the tissue in your nose to swell (making it stuffy), your nose and eyes to run, and your eyes, nose, and sometimes mouth to itch. Sometimes, you may also get an itchy rash on your skin called hives.

Antihistamines reduce or block histamines, so they stop allergy symptoms.

These medicines work well to ease symptoms of different types of allergies, including seasonal (hay fever), indoor, and food allergies. But they can't relieve every symptom.

To treat nasal congestion, your doctor may recommend an OTC steroid nasal spray. They have few side effects and can really improve your stuffy nose. Like antihistamines, doctors usually recommend that you start them a few weeks before your allergy season. There’s also a prescription steroid and antihistamine nasal spray called azelastine (Dymista) that seems to improve allergy symptoms more than either drug when used alone.

If you’re really blocked up, they may suggest using a decongestant for a few days to reduce nasal swelling. But they’re not recommended for long-term use, as they can cause side effects such as high blood pressure. 

Eye drops treat symptoms of allergies, including itchy, watery eyes.

Types of Antihistamines

They come in different forms, including tablets, capsules, liquids, gels, creams, skin sprays, nasal sprays, and eye drops. Some are only available by prescription, while others you can buy over the counter at your local pharmacy. Second-generation antihistamines don’t make you sleepy, while first-generation antihistamines do.

OTC antihistamines

There are many different oral antihistamines available over the counter. There are two main types.

First generation antihistamines. Also known as sedating antihistamines, they include:

Chlorpheniramine (Chlor-Trimeton)

Clemastine (Tavist)

Diphenhydramine (Benadryl)

Second generation antihistamines. These don’t make you sleepy:

Other OTC options include the antihistamine nasal spray azelastine (Astepro) and eye drops such as ketotifen (Alaway) and olopatadine (Pataday). Some medications combine an antihistamine and a decongestant (such as pseudoephedrine) to ease congestion. These require you to have a photo ID to purchase and are stored behind the pharmacy counter.

Prescription antihistamines

The following pills, eye drops, and nasal sprays are available with a doctor’s prescription: 

Azelastine eye drops

Azelastine nasal spray (Astelin)

Carbinoxamine (RyVent, Karbinal ER)

Cyproheptadine

Desloratadine (Clarinex)

Hydroxyzine (Vistaril)

The best antihistamines are ones that last 12-24 hours and don’t make you sleepy. Look for ones that contain:

Cetirizine

Fexofenadine

Levocetirizine

Loratadine

First-generation drugs, such as diphenhydramine, also work well. But they can make you very tired, so you should only use them at night before you go to sleep. Older adults should avoid them since they are more likely to develop symptoms such as dry mouth and eyes, trouble peeing, and confusion.

You can also save money if you buy generic antihistamines. They contain the exact same ingredients as name-brand ones. They work just as well and are just as safe.

First-generation antihistamines tend to cause more side effects, particularly drowsiness.

Second-generation antihistamines have fewer side effects, so they may be a better choice for some people.

Some of the main side effects of antihistamines include:

Dry mouth

Drowsiness

Dizziness

Nausea and vomiting

Restlessness or moodiness (in some children)

Trouble peeing or not being able to pee

Blurred vision

Confusion

Dry eyes

Weight gain

Impotence

Feeling hyper (usually occurs only in kids)

Memory problems

If you take an antihistamine that causes drowsiness, do so before bedtime. Don’t take it during the day before you drive or use machinery. If you take a newer antihistamine, don’t drive or operate machinery until you know how the antihistamine affects you. Some of these drugs have also been shown to affect how you drive.

Read the label before you take an allergy drug. Antihistamines may interact with other medications you are taking.

Talk to your doctor first if you have an enlarged prostate, heart disease, high blood pressure, thyroid problems, kidney or liver disease, a bladder obstruction, or glaucoma. Also, check with your doctor if you are pregnant or nursing. 

Kids under the age of 2 shouldn’t be given first-generation antihistamines. If they are between 2 and 6, talk to their pediatrician. These drugs have been linked to seizures, can hype them up, and have also been linked to poor school performance.

Don’t drink alcohol when you take antihistamines. The combination can make you sleepy.

Antihistamines help to relieve certain allergy symptoms such as itchiness, sneezing, and a runny nose. They’re less effective for other symptoms, such as a stuffed-up nose. You can buy antihistamines over-the-counter or have your doctor write you a prescription. People usually take them as pills, but they’re also available as nasal sprays, eye drops, and skin creams. Newer antihistamines, known as second-generation antihistamines, work well and have fewer side effects than older ones.

Are antihistamines safe during pregnancy?

In general, OTC antihistamines are considered safe during pregnancy. There are studies to show that certain older ones, such as chlorpheniramine (Chlor-Trimeton, GoodSense Allergy Relief) and dexchlorpheniramine (RyClora), don’t harm your baby. Newer ones, such as cetirizine and loratadine, are also thought to be safe. Always check with your doctor before you take any allergy medication. You should also avoid products that contain decongestants such as pseudoephedrine, which has been linked to abdominal wall birth defects when taken in the first trimester of pregnancy.

Which antihistamine is best for allergies?

There’s no one antihistamine that’s best for allergies. But in general, doctors recommend that you start with a nonsedating antihistamine, such as loratadine (Claritin, Alavert), desloratadine (Clarinex), cetirizine (Zyrtec), levocetirizine (Xyzal) and fexofenadine (Allegra). 

Do antihistamines work for all allergies?

Since antihistamines block histamine, they help stop symptoms of all different types of allergies, including indoor, outdoor, and food allergies. But they don’t work for all symptoms. If you have a very stuffy nose, your doctor may suggest a steroid nasal spray and possibly even short-term use of a decongestant. 

What is the best allergy medicine for adults that is nondrowsy?

Any of the newer antihistamines, such as loratadine (Claritin, Alavert), desloratadine (Clarinex), cetirizine (Zyrtec), levocetirizine (Xyzal) and fexofenadine (Allegra), shouldn't make you feel sleepy.