Why do some antihistamines make you drowsy?

Allergies like hay fever can cause uncomfortable symptoms like itchy watery eyes, sneezing, blocked or runny noses. But choosing an antihistamine to relieve your symptoms can be tricky, particularly when some antihistamines can impact your day by causing drowsiness. But why do different antihistamines cause drowsiness?

It’s because some antihistamines pass through the “blood brain barrier” – a unique, tightly packed, layer of cells that line the brain and spinal cord”. 1 This barrier reacts differently to different antihistamines which can result in drowsiness.

What is the Blood Brain Barrier & how does it work?

“The blood-brain barrier (BBB) is a unique, tightly packed, layer of cells that line the brain and spinal cord”. 1

The brain is extremely precious and needs to be protected – and this is where the BBB comes in: it helps protect the brain from harmful toxins and infection-causing bacteria, viruses or other microorganisms from entering. 2 However, the BBB is also a selective filter which allows essential nutrients/molecules to pass through to the brain.3,4 The BBB is made up of special cells, called endothelial cells, which line the blood capillaries. In the brain, these are much closer together4 -7, to create a physical barrier between the brain and the rest of the body 6,7 Another important part of the BBB are astrocytes, another type of cell which help to regulate the BBB function.8.9 The function of the BBB is important as different antihistamines will either pass through the barrier, causing drowsiness, or be prevented from entering, leaving you to enjoy your day.

Why is the BBB important for allergy sufferers and their treatment?

­­Different generations of antihistamines will either pass through the BBB, causing drowsiness, or be prevented from crossing so you can go about your day 14

First-generation antihistamines, like chlorphenamine, were developed back in the 1940s and have faithfully been providing symptom relief to millions of people ever since.15 They are characterised by their ability to block the action of histamine, which is heavily involved in the body’s allergic response.16 However, first-generation antihistamines pass through the BBB and can cause side effects such as drowsiness and impair cognitive function.18,20

“First-generation antihistamines cross the blood-brain barrier, and can cause drowsiness” 19

What are the benefits of second-generation antihistamines like Allevia®?

Second-generation antihistamines like Allevia® were developed to selectively target allergy symptoms without causing drowsiness in most people.

In comparison to first-generation antihistamines, second-generation antihistamines like Allevia®, were developed to more selectively target H1 receptors – which are responsible for many of the symptoms of allergies.18

“The discovery of antihistamines that selectively act on H1 receptors, currently called second generation drugs, could be considered the greatest breakthrough in more than 70 years” 18

Unlike first-generation antihistamines, second-generation histamines do not cross the BBB in most people and therefore do not cause drowsiness or affect brain function.14,18,20 That said, a minority of consumers can still experience drowsiness with second generation antihistamines like Allevia®.20

Allevia®  is a second-generation antihistamine. Unlike first generation antihistamine, second-generation antihistamines helps relieve allergy symptoms and does not cause drowsiness in most people.14-20 Don’t let pollen get in the way, live your greatness with Allevia 120mg tablets for the symptomatic relief of allergic rhinitis. Contains fexofenadine. Always read the label.

MAT-XU-2302108 (v1.0) May 2023


  1. Multiple sclerosis trust. Blood-Brain barrier. Available at:  https://mstrust.org.uk/a-z/blood-brain-barrier Accessed: March 2023.
  2. The University of Queensland. What is the blood-brain barrier: Available at: https://qbi.uq.edu.au/brain/brain-anatomy/what-blood-brain-barrier Accessed: March 2023.
  3. Knox, E.G., Aburto, M.R., Clarke, G. et al. The blood-brain barrier in aging and neurodegeneration. Mol Psychiatry. 2022;27,2659-2673.
  4. Lochhead JJ, Yang J, Ronaldson PT, Davis TP. Structure, Function, and Regulation of the Blood-Brain Barrier Tight Junction in Central Nervous System Disorders. Front Physiol. 2020;11:914.
  5. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Blood Vessels and Endothelial Cells. Available at: https://www.ncbi.nlm.nih.gov/books/NBK26848/ Accessed: March 2023.
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  12. American Society for Microbiology. How Pathogens Penetrate the Blood-Brain Barrier. Available at: https://asm.org/Articles/2020/April/How-Pathogens-Penetrate-the-Blood-Brain-Barrier  Accessed: March 2023.
  13. Bayir E, Sendemir A. Role of Intermediate Filaments in Blood-Brain Barrier in Health and Disease. Cells. 2021;10(6):1400
  14. Farzam K, Sabir S, O'Rourke MC. Antihistamines. [Updated 2022 Jul 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022. Available at: https://www.ncbi.nlm.nih.gov/books/NBK538188/ Accessed:  March 2023.
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  20. Church MK, Church DS. Pharmacology of antihistamines. Indian J Dermatol. 2013;58(3):219-24.

Take Allevia before your symptoms take over you

Life isn’t waiting for your allergy symptoms to pass and neither should you.

That’s why Allevia® helps deliver relief that lasts 24 hours, acts within one hour and is prescription strength, so you can get on with enjoying your day.​