Hay Fever and Antihistamines
For millions of people around the world, pollen season means weeks of sneezing, itchy eyes and a blocked or runny nose. 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?
How Antihistamines Work
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.
What Does the Research Say?
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.
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. A 2024 study also found that dementia risk appeared to increase with higher cumulative doses of antihistamines. The association was stronger for first-generation medicines but was also seen, to a lesser extent, with newer second-generation antihistamines.
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. 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 systemic inflammation may play a role in the biological processes that contribute to dementia. These overlapping factors make the studies difficult to interpret.
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.
- Antihistamines are not the only treatment available. Steroid nasal sprays can also help to reduce inflammation inside the nose.
- For people with severe hay fever that does not improve with standard treatment, allergen immunotherapy may be an option.