Why Some Antihistamines Make You Sleepy

Antihistamines are a blessing for many people, but why do they make you sleepy?
Why Some Antihistamines Make You Sleepy
Diego Pereira

Reviewed and approved by el médico Diego Pereira.

Last update: 25 January, 2023

Sleep is one of the main side effects of taking antihistamines. In fact, and as specialists point out, many people choose to use them to deal with their sleeping problems. Have you ever wondered why some antihistamines make you sleepy and others don’t? Let’s see the reasons behind it according to the scientists.

Antihistamines and allergies

Antihistamines are a group of drugs used to treat histamine-mediated conditions. Histamine is an endogenous chemical messenger that is produced to deal with an allergen.

It’s released by some white blood cells and tissue cells, and it binds to a histamine receptor on other cell groups. In the process, it causes a higher level of vascular permeability, which has several physiological consequences.

For example, it encourages fluid to move from capillaries into surrounding tissues. This causes swelling and dilation of the vessels, nasal congestion, runny nose, sneezing, and many other typical allergy symptoms. Histamine doesn’t only fulfill this function in the body. As experts point out, it has a role in regulating various physiological and behavioral functions.

Among other things, it’s involved in arousal, fluid balance, stress, learning and memory, pain perception, thermoregulation, and various neuroendocrine processes. Faced with all this, antihistamines stop the process by acting as histamine receptor antagonists. There are two types: those that act on H-1 receptors and those that act on H-2 receptors.

In principle, drugs that bind to H-1 receptors are used to treat allergies and allergic rhinitis. For their part, those that adhere to H-2 receptors are used to address gastrointestinal problems, such as acid reflux, gastritis, and peptic ulcers.

There are other histamine receptors, known as H-3 and H-4, but there are no clinical benefits associated with antagonists acting on them.

Why do antihistamines make you sleepy?

Antihistamines make you sleepy.
As early antihistamines became more widely used, experts became aware of their effects on sleep.

The feeling of sleepiness after taking antihistamines is something that was discovered during the second half of the 20th century. We have already explained to you that there are two types of antihistamines, depending on the receptors they act on (H-1 and H-2). Now, antihistamines that act on H-1 receptors are classified according to generation.

These are the most widely used, as they have been approved to treat allergic conjunctivitis, urticaria, sinusitis, atopic dermatitis, angioedema, and the aforementioned allergic rhinitis. All this introduction is useful for us to affirm the following: first-generation H-1 receptor antihistamines have a greater impact on sleep. Let’s see the reasons and how they contrast with those of other types.

First generation antihistamines

First-generation antihistamines don’t only act on H-1 receptors as antagonists, but in the process also cross the blood-brain barrier into the central nervous system (CNS). As the experts point out, this generates a series of adverse effects. The most important are the sedative effects and the temporary deterioration of cognitive and psychomotor function.

Its average duration in the system is 4 to 6 hours, because this time the adverse effects disappear. Until recently they could be accessed as an over-the-counter medicine, but given their potential sedative effect, in many countries they have been regulated to only be purchased by prescription.

Evidence has pointed the finger at these drugs in plane, car, and boating accidents due to their sedative effect. Some of these are the following:

  • Hydroxyzine
  • Chlorpheniramine
  • Doxepin
  • Diphenhydramine
  • Promethazine

In general, scientists advise against the use of first-generation antihistamines for two reasons: their sleep-inducing effects and because they are less effective than second-generation drugs.

Second generation antihistamines

Antihistamines and sleep are a recurring problem
Nowadays, second-generation antihistamines are much more popular than first-generation ones.

Most second-generation H-1 antihistamines have a duration of action of at least 24 hours. In addition to having a longer effect, they don’t cross the blood-brain barrier as easily as the former. Of all of them, cetirizine is known to be the most likely to cause sedation, although only in high doses. Those available in the current market are the following:

  • Cetirizine
  • Levocetirizine
  • Desloratadine
  • Loratadine
  • Fexofenadine

Some of these are also known as third-generation antihistamines, although the term new – generation antihistamines is generally preferred, in order to distinguish them from the former. So, you now know why some antihistamines make you sleepy and why they’re difficult to access without a prescription.

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