December 9, 2022

Antihistamine drugs: types and side effects


  • Antihistamines are drugs that treat the symptoms of allergic reactions such as 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 tips.

Thus, your allergies reappear. Your eyes are itchy and inflamed, your nose is runny, and you can’t stop sneezing. If this sounds like you, an antihistamine can help.

Antihistamines are a type of medicine that neutralizes histamine, a naturally occurring chemical released in the body during an allergic reaction that causes itching and inflammation.

For example, antihistamines like Claritin, Allegra, and Zyrtec are most often used to treat symptoms of

seasonal allergies
or hay fever.

These standard antihistamines are available over the counter as oral pills, eye drops, and nasal sprays, but doctors may also prescribe stronger antihistamines. Here’s what you need to know about antihistamines.

Types of antihistamines

According to American Academy of 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 in the 1950s, the first generation of antihistamines were in use.

These first generation antihistamines usually cause drowsiness because they interrupt the circadian rhythms that control your normal feeling of drowsiness.

In the 1980s, the second generation of antihistamines was created. They were less sedating, or not sedating at all, because they targeted the histamine reaction better without affecting drowsiness.

Today, the two forms still exist, for different purposes. For example, you might want a sedating antihistamine like Benadryl if your allergy symptoms are preventing you from sleeping, says the NHS. However, antihistamines should not be used to help sleep on their own – repeated studies have shown them to be ineffective in treating insomnia.

Overall, you should not take first generation antihistamines during the day, as drowsiness can impact tasks such as using machines or driving. A Study 2019 on the impact of first generation sleepy antihistamines on pilots found that the drug PAX Noche® reduced performance on a standard take-off and landing simulation by 11%, and up to 23% in an advanced take-off simulation and landing.

Antihistamines without drowsiness

Most of the time, you’ll want to take a non-drowsy antihistamine to reduce allergies that arise during the day.

Here are some common examples of antihistamines without drowsiness:

  • Cetirizine, such as Zyrtec
  • Desloratadine, like Clarinex
  • Fexofenadine, like Allegra
  • Loratadine, like Claritin

Antihistamines without drowsiness are recommended for their safety and effectiveness. These antihistamines take between one to two hours to take effect, and a 2005 study found that cetirizine and fexofenadine appeared within an hour. The four antihistamines relieve 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 drowsiness and are more suitable for daytime use, but they are still likely to cause some level of drowsiness,” says Ramzi yacoub, Pharm.D. and head of pharmacy at SingleCare.

Side effects

Before using an antihistamine, it is important to read the leaflet that comes with your medicine carefully and understand what you are taking.

Some common side effects for those taking first generation antihistamines are:

  • Drowsiness
  • Delayed response and coordination times
  • Have a dry mouth
  • Difficulty urinating

Second generation antihistamines have common side effects, too much:

  • Drowsiness (although less severe)
  • Have a dry mouth
  • Headache
  • Nausea

Yacoub recommends paying close attention to the dosage, as taking too much will increase the risk of negative side effects.

In addition, some people should not take antihistamines, including:

  • Pregnant and breastfeeding women
  • People taking certain types of medicines, such as antidepressants or cold and flu medicines that already contain antihistamines
  • People with epilepsy
  • Children under 6 – talk to a doctor or pharmacist about the best options for children

As with any medication, it’s important to talk to a healthcare professional who knows your medical history to get the most effective and safe treatment.

Nasal sprays

Nasal sprays are another form of antihistamine that particularly targets the symptoms of allergic rhinitis: itchy nose, sneezing, and congestion.

Types of antihistamine nasal sprays understand:

  • Azelastine, like Astepro
  • Olopatadine, like 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 are particularly irritating to the eyes and do not respond to oral antihistamines, antihistamine eye drops may be more effective for you.

Types of antihistamine eye drops understand:

  • Ketotifen, like Alaway
  • Olopatadine, like Pataday
  • Pheniramine and naphazoline, such as Visine

A Study 2019 found that antihistamine eye drops were effective in quickly reducing allergy symptoms. However, the same was true for other types of eye drops, such as liposomal eye spray, a non-pharmacological eye spray used for dry eyes. For both drugs, 80% of patients in each test group experienced some relief from their symptoms within two minutes.

Overall, it’s important to talk to your doctor first about the different options for relieving your allergy symptoms.

Insider takeaways

If your allergies are difficult to control, you can try different types of antihistamines to see what helps resolve your symptoms.

However, people with more severe allergies may find over-the-counter options not strong enough and should consult their doctor about prescription antihistamines or other medications like decongestants.