Antihistamines: Types, Works and Side Effects

Picture of antihistamines medication

Antihistamines are a type of medicine often used to treat a number of allergic health conditions.

Although antihistamines can’t cure these conditions, they often provide relief from symptoms. For example, antihistamines may be used to treat:

  • hay fever
  • allergic rhinitis- inflammation of the nose caused by an allergic reaction to substances such as dust mites
  • allergic skin conditions, such as eczema or urticaria (hives)
  • allergic conjunctivitis- inflammation of the eyes
  • allergic reactions caused by insect bites or stings
  • mild or moderate allergic reactions caused by food allergies – more severe allergic reactions (anaphylaxis) usually require emergency treatment with adrenaline

Antihistamines also have a number of other uses, such as treating stomach ulcers, insomnia (problems falling asleep) and motion sickness.

Antihistamines are available as

  • tablet or capsules (oral antihistamines)
  • creams, lotions and gels (topical antihistamines)
  • a nasal spray

Many antihistamines are available over the counter at a pharmacy, although some require a prescription.

What Types of Antihistamines Are Available?

Antihistamines come in different forms, including tablets, capsules, liquids, nasal sprays, and eyedrops. Some are only available by prescription. Others you can buy over the counter (OTC) at your local pharmacy.

Examples of prescription antihistamines include:

  • Astelin, Astepro (azelastine) nasal sprays
  • Atarax, Vistaril (hydroxyzine)
  • Clarinex (desloratadine)
  • Cyproheptadine (generic only)
  • Emadine (emadastine) eyedrops
  • Livostin (levocabastine) eyedrops
  • Optivar (azelastine) eyedrops
  • Palgic (carbinoxamine)
  • Xyzal (levocetirizine)

Examples of OTC antihistamines:

  • Allegra (fexofenadine)
  • Benadryl (diphenhydramine)
  • Dimetane (brompheniramine)
  • Claritin, Alavert (loratadine)
  • Tavist (clemastine)
  • Chlor-Trimeton (chlorpheniramine)
  • Zyrtec (certirizine)

Eyedrops likes Emadine and Livostin treat symptoms of eye allergies, including itchy, watery eyes. Some medications, like Allegra-D, Claritin-D, and Zyrtec-D, combine an antihistamine and a decongestant to relieve congestion.

How do antihistamines work?

When your body is exposed to allergens, it releases histamines. Histamines attach to the cells in your body and cause them to swell and leak fluid. This can cause itching, sneezing, runny nose, and watery eyes. Antihistamines prevent histamines from attaching to your cells and causing symptoms.

First-generation antihistamines also work in the part of the brain that controls nausea and vomiting. This is why they can help prevent motion sickness. Because 1 of the most common side effects of first-generation antihistamines is feeling sleepy, they are sometimes used to help people who have trouble sleeping (insomnia).

How to take antihistamines

Depending on your symptoms, you can take antihistamines:

  • Every day, to help keep daily symptoms under control
  • Only when you have symptoms
  • Before being exposed to things that often cause your allergy symptoms, such as a pet or certain plants

For many people with allergies, symptoms are the worst around 4 a.m. to 6 a.m. Taking an antihistamine at bedtime may help you or your child feel better in the morning during allergy season.

Most people who take antihistamines do not have any serious side-effects. If side-effects do occur, they are usually minor. The most common are:

  • Drowsiness
  • Headache
  • Dizziness
  • Agitation
  • Dry mouth
  • Blurred vision
  • Difficulty passing urine (urinary retention)
  • Stomach and gut upsets (gastrointestinal discomfort)

Various other medicines sometimes interact with antihistamines. For example, some antidepressants and some antifungal medicines. Therefore, if you are taking other medication, before taking an antihistamine check with your doctor or pharmacist to see if there is a risk of an interaction. If you are taking an antihistamine you should avoid alcohol, as this may make drowsiness worse.

For a full list of all the side-effects and possible interactions associated with your medicine, consult the leaflet that comes with your medication.

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