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Drowsy (sedating) antihistamines: uses and side effects

Dr Roger Henderson
Reviewed by Dr Roger HendersonReviewed on 13.10.2023 | 4 minutes read
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Antihistamines are a group of medications that prevent the level of histamine from rising in our body and reduce the symptoms this can cause. They can be used to treat a range of conditions including allergic reactions, motion sickness and insomnia. However, the term "antihistamines" most commonly refers to medications that are used to treat hay fever.

Antihistamines can be classified as sedating (drowsy) or non-sedating. With drowsy antihistamines, care should be exercised when driving or operating heavy machinery, and alcohol should be avoided. Below, Healthwords pharmacists will go into more detail about the different antihistamines and their side effects.

Doctor’s advice

Who should take antihistamines

Individuals experiencing allergic reactions, resulting in the following symptoms, can benefit from antihistamine use:

As well as for use in these symptoms:

If you find that you are getting regular allergic reactions and do not know the reason why, you should discuss with your doctor, and you may benefit from getting allergy testing.

Sedating antihistamines

Antihistamines, particularly the older variants, have a tendency to induce drowsiness, presenting both advantages and drawbacks. This drowsiness can be beneficial when symptoms are more pronounced at night, offering potential relief. However, it becomes a concern if tasks like operating heavy machinery or undertaking long drives are on the agenda. Notable examples of older antihistamines include chlorphenamine (present in Piriton) and promethazine (found in Phenergan).

These older antihistamines typically have a shorter duration of action, lasting around 4 to 6 hours. This shorter window of effectiveness may necessitate multiple daily doses or selective usage during specific periods, such as early mornings and evenings when pollen counts tend to be higher. Despite this limitation, some individuals find that these older antihistamines work more effectively for their particular hay fever symptoms. The process of identifying the most suitable treatment often involves a degree of trial and error.

It's worth noting that certain antihistamines causing drowsiness, like diphenhydramine, are also present in over-the-counter sleeping tablets. This intersection highlights the potential overlap in formulations designed to address both allergy symptoms and sleep-related issues, providing additional options for individuals seeking relief in these areas.

Why do antihistamines make you sleepy?

Sedating antihistamines, unlike their non-sedating counterparts, have the ability to penetrate the blood-brain barrier more readily. This allows them to impact the central nervous system and exert a sedative effect. The sedating effect is primarily attributed to their influence on the histamine receptors in the brain, causing drowsiness and affecting cognitive functions such as coordination and reaction speed.

On the other hand, non-sedating antihistamines are designed to have limited penetration of the blood-brain barrier. This targeted approach allows them to primarily block histamine receptors in peripheral tissues without significant effects on the central nervous system. Consequently, non-sedating antihistamines are less likely to cause drowsiness and are generally considered more suitable for activities requiring alertness, such as driving or operating machinery.

Non-sedating antihistamines

Non-sedating antihistamines represent a newish approach to managing allergic conditions, offering relief from symptoms without causing significant drowsiness. Unlike their older counterparts, these antihistamines are designed to minimise sedative effects, making them suitable for individuals who need to remain alert and focused throughout the day, even when experiencing allergy symptoms. Examples of non-sedating antihistamines include cetirizine (found in Zyrtec), loratadine (present in Clarityn), and fexofenadine (found in Allegra).

These newer antihistamines have a longer duration of action, typically lasting 24 hours with a once-daily dosing schedule. This extended effectiveness provides continuous relief, reducing the need for frequent dosing throughout the day. Non-sedating antihistamines are particularly favoured for individuals with active lifestyles or those engaged in tasks that require concentration. It's important to note that, while they are less likely to cause drowsiness, individual responses can vary, and it's advisable to monitor how the body reacts to these medications.

The choice between sedating and non-sedating antihistamines often depends on factors such as the nature of symptoms, the time of day when relief is needed, and an individual's daily activities. Consulting with a healthcare professional can help determine the most suitable antihistamine based on personal health considerations and lifestyle preferences.

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Dr Roger Henderson
Reviewed by Dr Roger Henderson
Reviewed on 13.10.2023
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