Antihistamines

What Is an Antihistamine?

Julie Lynn Marks
By Medically Reviewed by 
Reviewed:
Medically Reviewed

By blocking the effects of histamines, this class of drugs can treat allergies and many other ailments.

An antihistamine is a type of medicine used to treat common allergy symptoms, such as sneezing, watery eyes, hives, and a runny nose.

According to the American College of Allergy, Asthma, & Immunology, nasal allergies affect about 50 million people in the United States.

Certain antihistamines are also sometimes used to treat motion sickness, nausea, vomiting, dizziness, cough, sleep problems, anxiety, and Parkinson's disease.

The drugs work by blocking the effects of histamine, a substance in the body that can cause allergy symptoms.

Antihistamines come in different forms, such as capsules, tablets, liquids, eye drops, injections, and nasal sprays.

They can be purchased over-the-counter (OTC) or given as a prescription.

Some antihistamines are taken daily, while others are used only when symptoms occur.

Types of Antihistamines

Some common antihistamines include:

Antihistamine Side Effects

Common side effects of antihistamines include:

  • Drowsiness or sleepiness
  • Dizziness
  • Dry mouth, nose, or throat
  • Increased appetite and weight gain
  • Upset stomach
  • Thickening of mucus
  • Changes in vision
  • Feeling nervous, excited, or irritable

Antihistamine Precautions

Before taking an antihistamine, tell your doctor about all medical conditions you have, especially:

Don't drive or perform activities that require alertness until you know how the antihistamine you're taking affects you.

Follow the instructions on your prescription or package label carefully when taking an antihistamine. Don't take more of the medicine than is recommended.

Tell your doctor about all prescription, non-prescription, illegal, recreational, herbal, nutritional, or dietary drugs you're taking before starting on an antihistamine.

You may need to avoid grapefruit and grapefruit juice while taking an antihistamine, as they can affect how these drugs work in your body. Talk to your doctor if this is a concern.

Antihistamines and Alcohol

Alcohol may worsen certain side effects of antihistamines.

Avoid drinking alcohol while taking these medicines.

Antihistamines and Pregnancy

Tell your doctor if you're pregnant, or might become pregnant, while using an antihistamine.

You'll have to discuss the risks and benefits of taking the medicine during pregnancy.

Also, talk to your healthcare provider before taking antihistamines if you're breastfeeding.

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