Which antihistamines make you drowsy?

Older antihistamines—called first-generation antihistamines—may cause drowsiness, fatigue, and decreased alertness or concentration. These side effects are called sedation. Examples of these drugs include:

• Benadryl (diphenhydramine)
• Chlor-Trimeton (chlorpheniramine)

Newer antihistamines are called second-generation antihistamines. They are less likely to cause sedation. Examples include:

• Zyrtec (cetirizine)
• Xysal (levocetirizine)
• Claritin (laratodine)
• Clarinex (desloratadine)
• Allegra (fexofenadine)

Histamine is a natural substance of the immune system. It causes inflammation that can be an important part of your body’s defense against invaders like germs or viruses. Overreaction to histamine causes allergy symptoms and allergic reactions. All antihistamines block the effects of histamine, but first-generation antihistamines cross from the blood into the brain, called the blood-brain barrier. Because histamine also acts in the brain to help nerve cells communicate, antihistamines that cross the blood-brain barrier cause sedation.

Second-generation antihistamines are much less likely to cross the blood-brain barrier. Your doctor or pharmacist may suggest a second-generation antihistamine if you are treating allergy symptoms like a runny nose or an itchy rash.

However, if you are taking an antihistamine to treat motion sickness, relieve anxiety or use as a sleep aid, a first-generation antihistamine may be recommended due to the sedating effects.

There are many brands of antihistamines that are available over the counter (OTC), and finding the appropriate one for your needs can be confusing. Your health care provider may prescribe an antihistamine based on the symptoms or condition being treated. If you are taking an OTC antihistamine on your own, ask your pharmacist if it may cause sedation.