Millions of people take antihistamines for allergies, but some of these medicines can make you sleepy, slow your reactions and affect your concentration. Others are much less likely to have these effects. Knowing the difference can help you choose an allergy treatment that won’t interfere with your day.
By Dipa Kamdar for The Conversation
Antihistamines work by blocking histamine, a chemical released by the immune system during an allergic reaction. Histamine causes many common allergy symptoms, including itching, sneezing, watery eyes and a runny nose.
The first antihistamines were developed in the 1940s. Medicines such as chlorphenamine, diphenhydramine and promethazine are known as first-generation antihistamines, and they are still widely available in allergy medicines, cold remedies and sleep aids.
In the 1980s and 1990s, newer antihistamines such as loratadine, cetirizine and fexofenadine were introduced. Known as second-generation antihistamines, they were designed to cause fewer side-effects, especially drowsiness.
The reason some antihistamines cause drowsiness while others do not comes down to what happens in the brain. Older antihistamines can easily cross the blood-brain barrier, which helps control what enters the brain from the bloodstream. There, they block histamine receptors that help keep you alert and awake.
Brain imaging studies show that older antihistamines block more histamine receptors in the brain than newer ones. This is linked to a greater risk of sleepiness, slower reaction times and difficulty concentrating.
Newer antihistamines generally have less effect on the brain, although there are differences between medicines. One study found that fexofenadine occupied around 2% of histamine receptors in the brain, compared with 24% for cetirizine (a second-generation drug) and 50 to 60% for chlorpheniramine. This helps explain why some “non-drowsy” antihistamines may still cause sleepiness in certain people.
Researchers increasingly recognise that antihistamines do not fall neatly into two groups: those that cause sleepiness and those that do not. Instead, they are better viewed as existing on a spectrum.
Some older antihistamines have a strong effect on alertness; some newer medicines have a mild effect and others have very little effect on the brain at all. This is why two medicines that treat the same allergy symptoms can leave you feeling quite different.
Some people find the sedative effects useful. Diphenhydramine, for example, is sold in several over-the-counter sleep aids. But feeling sleepy is not always harmless.
Antihistamines can affect attention, memory, coordination and reaction times. This can make activities such as driving or operating machinery less safe. You may not realise how much you are affected: even if you feel only slightly tired, your reactions may be slower and your concentration poorer.
Newer antihistamines generally have less effect on the brain, although there are differences between medicines. One study found that fexofenadine occupied around 2% of histamine receptors in the brain, compared with 24% for cetirizine (a second-generation drug) and 50 to 60% for chlorpheniramine. This helps explain why some “non-drowsy” antihistamines may still cause sleepiness in certain people.
Researchers increasingly recognise that antihistamines do not fall neatly into two groups: those that cause sleepiness and those that do not. Instead, they are better viewed as existing on a spectrum.
Some older antihistamines have a strong effect on alertness; some newer medicines have a mild effect and others have very little effect on the brain at all. This is why two medicines that treat the same allergy symptoms can leave you feeling quite different.
Some people find the sedative effects useful. Diphenhydramine, for example, is sold in several over-the-counter sleep aids. But feeling sleepy is not always harmless.
Antihistamines can affect attention, memory, coordination and reaction times. This can make activities such as driving or operating machinery less safe. You may not realise how much you are affected: even if you feel only slightly tired, your reactions may be slower and your concentration poorer.
Dipa Kamdar is an associate professor in pharmacy practice at Kingston University, UK.
This article was republished from The Conversation under a Creative Commons licence.
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