Which antihistamines make you drowsy?

Medically reviewed by Sally Chao, MD. Last updated on Sep 23, 2024.

Official Answer by Drugs.com

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.

Does Zyrtec cause weight gain?

A study has shown that Zyrtec (generic name: cetirizine) is associated with weight gain if taken over the long-term. This may occur because antihistamines can block histamine-1 (H-1) receptors which will increase your appetite. Zyrtec may also cause drowsiness in some people and lower their energy expenditure, causing weight gain.

What is the Benadryl hat man?

The Hat Man is a fictional figure that started appearing on certain internet sites in the early 2000s that has recently been referred to as the “Benadryl Hat Man” after reports of people hallucinating and sighting him after overconsumption or abuse of Benadryl. Most of these reports have appeared on Reddit or the image-based bulletin site 4chan. Continue reading

What is the max dose of diphenhydramine for adults?

The maximum oral dose of diphenhydramine for adults is generally reported as 100mg for a single dose or no more than 300mg in 24 hours, but this can vary depending on what condition diphenhydramine is being used for, the salt of diphenhydramine used (there are two salts available in the US diphenhydramine citrate and diphenhydramine chloride), the age of the person, how much diphenhydramine they have tolerated before, and if they are on any interacting medications. Taking too much diphenhydramine can be fatal.

Which antihistamines make you drowsy?

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