What taking an antihistamine does to your brain

Telegraph

03-03-2025

If you're blighted by hay fever or other allergies, you probably rely heavily on antihistamines – one of the best medications to treat them – to ease your watery eyes, runny nose and sneezing.
However, research suggests that the common drugs could be harmful for our brain health. Despite conflicting evidence on just how much of a risk they may pose, one Harvard professor recently confessed that he 'thinks twice about taking antihistamines' as 'theoretically, these drugs might increase the risk of dementia by blocking a particular brain neurotransmitter'.
People who take antihistamines to manage their allergy symptoms shouldn't be alarmed, however, says Dr Adrian Morris, a consultant allergist at the Surrey Allergy Clinic.
'They shouldn't stop taking the drugs and be left struggling away with terrible symptoms because they're too terrified of taking these drugs,' he says. However, it's worth being aware of the potential risk.
What are they and how do they work?
'Antihistamines are drugs used against allergies,' says Dr Morris.
The immune system releases histamine to protect against infections but, in people with allergies, the immune system overreacts and releases histamine in response to a supposed threat – such as pollen, animal hair or dust – when it doesn't need to.
Histamine causes the blood vessels to dilate (which fuels inflammation in the skin, throat and nose), acts on the nerves to trigger sneezing and itchy skin and triggers the glands to make mucus, causing the nose to run and the chest to get phlegmy, Dr Morris explains.
Antihistamines can prevent this reaction, if taken before coming into contact with a substance you're allergic to, or reduce symptoms if you take them afterwards.
How antihistamines affect your brain
Some antihistamines can cross the blood-brain barrier – a membrane that's supposed to protect harmful substances from getting in, Dr Morris explains.
Older versions of antihistamines such as chlorphenamine, known as a first-generation antihistamines, are known to cross into the brain, while the newer, second generation antihistamines are less likely to, though it's still possible for them to accumulate in the organ, he says.
In the brain, antihistamines can block neurotransmitters (chemical messengers), including acetylcholine which is needed for learning and memory. They can also increase brain inflammation. 'This has the net effect of increasing the risk of dementia,' Dr Morris says.
It is known that people with Alzheimer's disease are known to lack acetylcholine and some research suggests that daily use of chlorphenamine for more than three years increases the risk of dementia – though other papers have not found this effect.
'It's not a clear cut difference but the more sedating an antihistamine is, the more it is going into the brain and could be a problem, especially if you take the drug for years on end,' Dr Morris says.
How to know which one to take
A wide range of antihistamines are available over the counter and contain varying active ingredients, explains Claire Nevinson, a superintendent pharmacist at Boots UK.
'These are divided into two groups based on the active ingredient – antihistamines that may cause drowsiness, which include the active ingredient chlorphenamine, and antihistamines that are less likely to cause drowsiness, which include cetirizine, fexofenadine, acrivastine and loratadine,' she says.
'The general rule now is to go for the second generation, non-sedating antihistamines and leave the older ones out of the equation,' Dr Morris says.
This is because, as well as causing drowsiness, the first generation pills are more likely to dilate the blood vessels, which can cause blood pressure to come down and trigger headaches and dizziness, he explains.
These sedating antihistamines can also cause dry mouth, blurred vision, difficulty urinating and constipation, Dr Morris says. They also only work for around four hours, while the newer drugs last for 24 hours, he notes.
It raises the question of why people still take the first generation antihistamines. 'Some people take them to help them to sleep at night,' he says. As they have a sedating effect, they can also help with feeling less nervous or anxious, which some people like, Dr Morris notes.
When it comes to choosing which second-generation antihistamine to take, cetirizine, loratadine and fexofenadine are the main ones that are recommended, he says.
Which one patients choose is personal preference but it can be helpful to change the drug they're taking every few months.
'What happens is tachyphylaxis – when you take a medication and it becomes less effective because the body becomes more tolerant to it,' Dr Morris says. 'I recommend patients to take one drug for a few months and then change as the drugs will work better.
'There's little evidence to suggest one antihistamine is better than any other at relieving allergy symptoms,' though the second generation drugs are less likely to have side effects that interfere with daily life, Nevinson says.
'There are a range of products available for hay fever relief and if you are ever unsure on how to best manage your hay fever symptoms, speak to your pharmacist who may be able to help,' she adds.
Other uses of antihistamines
Insect bites and stings
Insect bites and stings can trigger the body's immune system to release histamine, which in turn causes swelling and itchiness at the site of the bite. Antihistamines can ease this reaction, making bites less irritating.
Itchy skin
'As you get older, your skin gets drier and often it starts to itch, especially among men,' Dr Morris says. A daily antihistamine can relieve itching for these patients, he says.
Travel sickness
A first generation antihistamine, called cinnarizine, is known to improve blood flow to the inner ear, which can reduce feelings of nausea and dizziness caused by travel sickness.
Verdict: Should I take antihistamines?
The research that has found a link between antihistamines and dementia isn't perfect and it doesn't suggest that 'everyone who takes antihistamines for six months is going to end up with dementia,' Dr Morris says.
'We don't have any high-quality, long-term studies – we've got to watch people over their entire lifetime [to properly determine the risk],' he notes.
'I would say that the second generation is preferable to the first generation, and if an antihistamine makes you feel tired and lethargic, or sedates you, then maybe choose another one that you don't find sedating,' he says, 'as it's probably crossing the blood-brain barrier much less.'
FAQs
What happens if you take too many antihistamine pills?
'If you accidentally take too many antihistamines, you should seek medical help, taking the medicine packet with you so that a healthcare professional can see which type of antihistamine you've taken,' Nevinson says.
However, Dr Morris says there is no risk from accidentally taking two or three antihistamine pills, instead of the usually recommended one pill, because the drugs are non-toxic.
Can you drink alcohol while taking antihistamines?
'It is recommended not to drink alcohol whilst taking an antihistamine, particularly if it's a type that makes you drowsy,' Nevinson says.
'Doing so can increase the chances of feeling drowsy and reduce co-ordination, reaction speed and judgement. Most foods and other drinks don't tend to affect most antihistamines, however always read the leaflet that comes with your medicine to make sure.'
Can you take antihistamines alongside ibuprofen and paracetamol?
'There are currently no known interactions between antihistamines being taken alongside ibuprofen or paracetamol,' Nevinson says.
'However, it is important to speak to your pharmacist before taking any new medicines available over the counter to check for suitability.'