For millions of people around the world, pollen season means weeks of sneezing, itchy eyes and a blocked or runny nose. The timing varies depending on where you live and which plants are in flower, but grass pollen is one of the most common triggers.
Hay fever, also known as seasonal allergic rhinitis, is an allergic reaction to airborne pollen. Many people manage their symptoms with antihistamines bought from a pharmacy. But recent headlines have raised a worrying question: could some of the medicines used to relieve hay fever symptoms increase the risk of dementia?
Antihistamines block histamine, a chemical released by the immune system during an allergic reaction. Histamine causes symptoms such as itching, sneezing and a runny nose.
Older, first-generation antihistamines, such as diphenhydramine and chlorphenamine, are more likely to cause drowsiness. Newer, second-generation antihistamines, such as cetirizine, loratadine and fexofenadine, are generally less sedating.
Some older antihistamines also reduce the activity of acetylcholine, a chemical messenger involved in attention, learning and memory. Medicines that block the action of acetylcholine are described as having anticholinergic effects.
These older medicines should be used cautiously, particularly in later life. They can cause drowsiness and concentration problems, increasing the risk of falls. People should also take care when driving if a hay-fever medicine makes them sleepy, as highlighted in recent reports.
What does the research say about dementia risk?
Some studies have found an association between prolonged use of medicines with strong anticholinergic effects and a higher risk of dementia. These include some treatments for depression, Parkinson’s disease and bladder problems, as well as certain older antihistamines.
There is a plausible reason for concern: acetylcholine plays an important role in memory and thinking. Some medicines used to treat symptoms of Alzheimer’s disease work by increasing the amount of acetylcholine available in the brain. Anticholinergic medicines reduce its activity.
One large observational study found that people with the highest exposure to strong anticholinergic medicines had a greater risk of dementia. But observational studies can identify patterns without proving that one factor causes another. People who take these medicines may differ from those who do not in other ways that affect their dementia risk. Some may have underlying health conditions, while others may have been prescribed medication for symptoms linked to the early stages of dementia.
A 2024 study of people with allergic rhinitis also found that dementia risk appeared to increase with higher cumulative doses of antihistamines, meaning the total amount taken over time. The association was stronger for first-generation medicines but was also seen, to a lesser extent, with newer second-generation antihistamines.
That finding was puzzling since second-generation antihistamines are less likely to cross the blood-brain barrier, the protective boundary separating the bloodstream from the brain. They also tend to have fewer anticholinergic effects.
Some scientists questioned whether the medicines themselves were responsible. Other factors, such as the severity of a person’s allergies, could help to explain the apparent link.
More recent evidence is reassuring. A nationwide study found no evidence that prolonged use of second-generation antihistamines increased dementia risk.
The research does not suggest that occasional use of a newer, less sedating antihistamine causes dementia.
Why the link is difficult to untangle
Hay fever itself may help to explain the apparent link between antihistamines and dementia. For example, hay fever can disrupt sleep, and poor sleep has separately been linked to dementia risk.
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A recent observational study also found an association between allergic rhinitis itself and Alzheimer’s disease in older adults.
One possible explanation is inflammation. Allergic rhinitis involves an immune response that may contribute to inflammation beyond the nose. Researchers are investigating whether this wider, or systemic, inflammation, may play a role in the biological processes that contribute to dementia.
These overlapping factors make the studies difficult to interpret. People with more severe hay fever may be more likely to take antihistamines regularly. They may also be more likely to experience disrupted sleep or inflammation. Researchers cannot always separate the effects of the medication from those of the condition it treats.
What should people with hay fever do?
The evidence does not suggest that people should stop treating hay fever. But it is worth checking which medicine you are taking.
Newer antihistamines are generally less likely to cause drowsiness and tend to have fewer anticholinergic effects. People who regularly take an older, sedating antihistamine should discuss their medication with a pharmacist or GP, particularly if they are older or take other medicines with anticholinergic effects. Anyone taking a prescribed medicine should seek medical advice before stopping or changing it.
Antihistamines are not the only treatment available. Steroid nasal sprays can also help to reduce inflammation inside the nose.
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For people with severe hay fever that does not improve with standard treatment, allergen immunotherapy may be an option. This involves giving the body carefully controlled doses of an allergen, such as grass pollen, to gradually reduce its sensitivity.
Treatment often begins several months before pollen season and may continue for years. Research using tens of thousands of medical records suggests that allergen immunotherapy can reduce hay-fever symptoms. In people who also have asthma, it has been associated with fewer severe asthma flare-ups and cases of pneumonia.
The picture is more nuanced than some headlines suggest. Older, sedating antihistamines have drawbacks, especially when used regularly by older people. Newer versions are generally preferred because they are less likely to cause drowsiness and tend to have fewer anticholinergic effects.
The practical message is simple: treat hay fever, but choose the treatment carefully. Anyone concerned about taking antihistamines regularly should ask a pharmacist or GP to review their options.
Eef Hogervorst has received funding from various governmental funding agencies including ESRC and NIHR, the Road Safety Trust and ISPF as well as charities such as Alzheimer's Research UK. She is affiliated with Loughborough University and acted as dementia advisor for the NICE Guidelines
