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- Antihistamines are medications that treat allergic reactions symptoms like dry eyes or sneezing.
- Antihistamines can be classified into two types: drowsy and non-drowsy.
- Common side effects of antihistamines include drowsiness, headache, and dry mouth.
- Visit Insider's Health Reference library for more advice.
So, your allergies are acting up again. Your eyes are itchy and inflamed, your nose is running, and you can't stop sneezing. If this sounds like you, an antihistamine may help.
Antihistamines are a type of medication that counteract histamine, a natural chemical released in the body during an allergic reaction that causes itchiness and inflammation.
For example, antihistamines like Claritin, Allegra, and Zyrtec are most commonly used to treat the symptoms of seasonal allergies or hay fever.
These standard antihistamines are available over-the-counter in the form of oral pills, eye drops, and nasal sprays – but stronger ones can also be prescribed by doctors. Here's what you should know about antihistamines.
Types of antihistamines
According to the American Academy for Allergy, Asthma, and Immunology (AAAAI), there are two types of antihistamines:
- First-generation, or drowsy antihistamines
- Second-generation, or non-drowsy antihistamines
Antihistamines were discovered in the 1930s by pharmacologist Daniel Bovet, and by the 1950s, the first generation of antihistamines was in use.
These first-generation antihistamines commonly cause drowsiness because they interrupt the circadian rhythms that control your normal feelings of sleepiness.
In the 1980s, the second generation of antihistamines were created. They were less sedative, or not sedative at all, as they were better at targeting the histamine reaction without affecting sleepiness.
Today, both forms still exist, for different purposes. For example, you may want a sedative antihistamine like Benadryl if your allergy symptoms are making it difficult for you to sleep, says the NHS. However, antihistamines shouldn't be used to help sleep alone - repeated studies have shown their inefficacy in treating insomnia.
Overall, you should not take first-generation antihistamines during the day, as drowsiness can impact tasks such as operating machinery or driving. A 2019 study on first-generation drowsy antihistamines' impact on pilots found that the medication PAX Noche® reduced performance on a standard takeoff and landing simulation by 11%, and up to 23% in an advanced takeoff and landing simulation.
Non-drowsy antihistamines
Most of the time, you'll want to take a non-drowsy antihistamine to reduce allergies that strike during the day.
Some common examples of non-drowsy antihistamines are:
- Cetirizine, such as Zyrtec
- Desloratadine, such as Clarinex
- Fexofenadine, such as Allegra
- Loratadine, such as Claritin
Non-drowsy antihistamines are recommended for their safety and effectiveness. These antihistamines take between one to two hours to take effect, and a 2005 study found both Cetirizine and Fexofenadine had onset within an hour. All four of the antihistamines provide relief from symptoms for 24 hours, stopping itching and sneezing, and allergic conjunctivitis.
However, even with non-drowsy antihistamines, drowsiness can be a side-effect, and they should be used with caution.
"Cetirizine, Desloratadine, and Fexofenadine cause less intense drowsiness and are better suited for daytime use, but they are still likely to cause a certain level of drowsiness," says Ramzi Yacoub, Pharm.D. and Chief Pharmacy Officer at SingleCare.
Side effects
Before using an antihistamine, it's important to thoroughly read the leaflet accompanying your medication and understand what you're taking.
Some common side effects for those taking first-generation antihistamines are:
- Drowsiness
- Delayed response time and coordination
- Having a dry mouth
- Trouble urinating
Second-generation antihistamines have some common side effects, too:
- Drowsiness (though less severe)
- Having a dry mouth
- Headaches
- Nausea
Yacoub recommends paying particular attention to the dosage, as taking too much will increase the likelihood of negative side-effects.
In addition, there are certain people who shouldn't take antihistamines, including:
- Pregnant and breastfeeding women
- People taking certain kinds of medication, such as antidepressants or cold and flu medication which contain antihistamines already
- People with epilepsy
- Children under the age of 6 - speak to a doctor or pharmacist about the best options for children
Like any medication, speaking with a medical professional who knows your medical history is important to get the most effective and safe treatment.
Nasal sprays
Nasal sprays are another form of antihistamine that particularly target the symptoms of allergic rhinitis: itching nose, sneezing, and congestion.
Types of antihistamine nasal sprays include:
- Azelastine, such as Astepro
- Olopatadine, such as Patanase
The most common side-effect of nasal sprays is a bitter taste, but they are generally safe and easy to use.
Eye drops
If you have allergies that particularly irritate the eyes and don't respond to oral antihistamines, then antihistamine eye drops might work better for you.
Types of antihistamine eye drops include:
- Ketotifen, such as Alaway
- Olopatadine, such as Pataday
- Pheniramine and naphazoline, such as Visine
A 2019 study found antihistamine eye drops were effective at quickly reducing allergy symptoms. However, so were other types of eye drops, such as liposomal eye spray, a nonpharmacological eye spray used for dry eyes. For both medications, 80% of patients in each test group experienced some relief for their symptoms within two minutes.
Overall, it's important to speak to your doctor first about different options to relieve your allergy symptoms.
Insider's takeaway
If your allergies are difficult to control, you may want to try out different types of antihistamines to see what helps resolve your symptoms.
However, those with more severe allergies may find that over-the-counter options aren't strong enough, and should consult with their doctor about prescription antihistamines, or other medications like decongestants.