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........

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